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2.2: Chapter 3 - What Is a Search? Some Specifics

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    In the material assigned for this chapter, we begin applying the rules set forth in Katz and Jones to specific activities. As the cases make clear, the word “search” for purposes of the Fourth Amendment does not have its normal English meaning, that is, something to the effect of “try to find something” or “look for something.” Instead, the Supreme Court has created a legal term of art. Some activities that one might normally describe with the word “search” (such as looking through someone’s garbage in the hope of finding something interesting) turn out not to count as “searches” in Fourth Amendment jurisprudence. Students should consider when reading these cases whether the Court’s reasoning is persuasive. Further, they should consider whether a unifying set of principles can be found that (at least most of the time) allows one to predict whether a given activity will count as a “search.” Absent such a set of principles, it may appear that the Court’s doctrine in this area is somewhat arbitrary.

    Supreme Court of the United States

    California v. Billy Greenwood

    Decided May 16, 1988—486 U.S. 35

    Justice WHITE delivered the opinion of the Court.

    The issue here is whether the Fourth Amendment prohibits the warrantless search and seizure of garbage left for collection outside the curtilage of a home. We conclude, in accordance with the vast majority of lower courts that have addressed the issue, that it does not.


    In early 1984, Investigator Jenny Stracner of the Laguna Beach Police Department received information indicating that respondent Greenwood might be engaged in narcotics trafficking. Stracner sought to investigate this information by conducting a surveillance of Greenwood’s home.

    On April 6, 1984, Stracner asked the neighborhood’s regular trash collector to pick up the plastic garbage bags that Greenwood had left on the curb in front of his house and to turn the bags over to her without mixing their contents with garbage from other houses. The trash collector cleaned his truck bin of other refuse, collected the garbage bags from the street in front of Greenwood’s house, and turned the bags over to Stracner. The officer searched through the rubbish and found items indicative of narcotics use. She recited the information that she had gleaned from the trash search in an affidavit in support of a warrant to search Greenwood’s home.

    Police officers encountered both respondents at the house later that day when they arrived to execute the warrant. The police discovered quantities of cocaine and hashish during their search of the house. Respondents were arrested on felony narcotics charges. They subsequently posted bail.

    The police continued to receive reports of many late-night visitors to the Greenwood house. On May 4, Investigator Robert Rahaeuser obtained Greenwood’s garbage from the regular trash collector in the same manner as had Stracner. The garbage again contained evidence of narcotics use.

    Rahaeuser secured another search warrant for Greenwood’s home based on the information from the second trash search. The police found more narcotics and evidence of narcotics trafficking when they executed the warrant. Greenwood was again arrested.


    The warrantless search and seizure of the garbage bags left at the curb outside the Greenwood house would violate the Fourth Amendment only if respondents manifested a subjective expectation of privacy in their garbage that society accepts as objectively reasonable. Respondents do not disagree with this standard.

    They assert, however, that they had, and exhibited, an expectation of privacy with respect to the trash that was searched by the police: The trash, which was placed on the street for collection at a fixed time, was contained in opaque plastic bags, which the garbage collector was expected to pick up, mingle with the trash of others, and deposit at the garbage dump. The trash was only temporarily on the street, and there was little likelihood that it would be inspected by anyone.

    It may well be that respondents did not expect that the contents of their garbage bags would become known to the police or other members of the public. An expectation of privacy does not give rise to Fourth Amendment protection, however, unless society is prepared to accept that expectation as objectively reasonable.

    Here, we conclude that respondents exposed their garbage to the public sufficiently to defeat their claim to Fourth Amendment protection. It is common knowledge that plastic garbage bags left on or at the side of a public street are readily accessible to animals, children, scavengers, snoops, and other members of the public. Moreover, respondents placed their refuse at the curb for the express purpose of conveying it to a third party, the trash collector, who might himself have sorted through respondents’ trash or permitted others, such as the police, to do so. Accordingly, having deposited their garbage “in an area particularly suited for public inspection and, in a manner of speaking, public consumption, for the express purpose of having strangers take it,” respondents could have had no reasonable expectation of privacy in the inculpatory items that they discarded.

    Furthermore, as we have held, the police cannot reasonably be expected to avert their eyes from evidence of criminal activity that could have been observed by any member of the public. Hence, “[w]hat a person knowingly exposes to the public, even in his own home or office, is not a subject of Fourth Amendment protection.”

    The judgment of the California Court of Appeal is therefore reversed, and this case is remanded for further proceedings not inconsistent with this opinion.

    Justice KENNEDY took no part in the consideration or decision of this case.

    Justice BRENNAN, with whom Justice MARSHALL joins, dissenting.

    Every week for two months, and at least once more a month later, the Laguna Beach police clawed through the trash that respondent Greenwood left in opaque, sealed bags on the curb outside his home. Complete strangers minutely scrutinized their bounty, undoubtedly dredging up intimate details of Greenwood’s private life and habits. The intrusions proceeded without a warrant, and no court before or since has concluded that the police acted on probable cause to believe Greenwood was engaged in any criminal activity.

    Scrutiny of another’s trash is contrary to commonly accepted notions of civilized behavior. I suspect, therefore, that members of our society will be shocked to learn that the Court, the ultimate guarantor of liberty, deems unreasonable our expectation that the aspects of our private lives that are concealed safely in a trash bag will not become public.


    “A container which can support a reasonable expectation of privacy may not be searched, even on probable cause, without a warrant.” Thus, as the Court observes, if Greenwood had a reasonable expectation that the contents of the bags that he placed on the curb would remain private, the warrantless search of those bags violated the Fourth Amendment.

    The Framers of the Fourth Amendment understood that “unreasonable searches” of “paper[s] and effects”—no less than “unreasonable searches” of “person[s] and houses”—infringe privacy. As early as 1878, this Court acknowledged that the contents of “[l]etters and sealed packages … in the mail are as fully guarded from examination and inspection … as if they were retained by the parties forwarding them in their own domiciles.” In short, so long as a package is “closed against inspection,” the Fourth Amendment protects its contents, “wherever they may be,” and the police must obtain a warrant to search it just “as is required when papers are subjected to search in one’s own household.”

    With the emergence of the reasonable-expectation-of-privacy analysis, see Katz v. United States, we have reaffirmed this fundamental principle. Accordingly, we have found a reasonable expectation of privacy in the contents of a 200–pound “double-locked footlocker,” a “comparatively small, unlocked suitcase,” a “totebag,” and “packages wrapped in green opaque plastic,”

    Our precedent, therefore, leaves no room to doubt that had respondents been carrying their personal effects in opaque, sealed plastic bags—identical to the ones they placed on the curb—their privacy would have been protected from warrantless police intrusion. So far as Fourth Amendment protection is concerned, opaque plastic bags are every bit as worthy as “packages wrapped in green opaque plastic” and “double-locked footlocker[s].”


    Respondents deserve no less protection just because Greenwood used the bags to discard rather than to transport his personal effects. Their contents are not inherently any less private, and Greenwood’s decision to discard them, at least in the manner in which he did, does not diminish his expectation of privacy.

    A trash bag, like any of the above-mentioned containers, “is a common repository for one’s personal effects” and, even more than many of them, is “therefore … inevitably associated with the expectation of privacy.” A single bag of trash testifies eloquently to the eating, reading, and recreational habits of the person who produced it. A search of trash, like a search of the bedroom, can relate intimate details about sexual practices, health, and personal hygiene. Like rifling through desk drawers or intercepting phone calls, rummaging through trash can divulge the target’s financial and professional status, political affiliations and inclinations, private thoughts, personal relationships, and romantic interests. It cannot be doubted that a sealed trash bag harbors telling evidence of the “intimate activity associated with the ‘sanctity of a man’s home and the privacies of life,’” which the Fourth Amendment is designed to protect.

    In evaluating the reasonableness of Greenwood’s expectation that his sealed trash bags would not be invaded, the Court has held that we must look to “understandings that are recognized and permitted by society.” Most of us, I believe, would be incensed to discover a meddler—whether a neighbor, a reporter, or a detective—scrutinizing our sealed trash containers to discover some detail of our personal lives. That was, quite naturally, the reaction to the sole incident on which the Court bases its conclusion that “snoops” and the like defeat the expectation of privacy in trash. When a tabloid reporter examined then-Secretary of State Henry Kissinger’s trash and published his findings, Kissinger was “really revolted” by the intrusion and his wife suffered “grave anguish.” The public response roundly condemning the reporter demonstrates that society not only recognized those reactions as reasonable, but shared them as well. Commentators variously characterized his conduct as “a disgusting invasion of personal privacy,” and contrary to “the way decent people behave in relation to each other.”

    Had Greenwood flaunted his intimate activity by strewing his trash all over the curb for all to see, or had some nongovernmental intruder invaded his privacy and done the same, I could accept the Court’s conclusion that an expectation of privacy would have been unreasonable. Similarly, had police searching the city dump run across incriminating evidence that, despite commingling with the trash of others, still retained its identity as Greenwood’s, we would have a different case. But all that Greenwood “exposed … to the public,” were the exteriors of several opaque, sealed containers. Until the bags were opened by police, they hid their contents from the public’s view every bit as much as did Chadwick’s double-locked footlocker and Robbins’ green, plastic wrapping. Faithful application of the warrant requirement does not require police to “avert their eyes from evidence of criminal activity that could have been observed by any member of the public.” Rather, it only requires them to adhere to norms of privacy that members of the public plainly acknowledge.

    The mere possibility that unwelcome meddlers might open and rummage through the containers does not negate the expectation of privacy in their contents any more than the possibility of a burglary negates an expectation of privacy in the home; or the possibility of a private intrusion negates an expectation of privacy in an unopened package; or the possibility that an operator will listen in on a telephone conversation negates an expectation of privacy in the words spoken on the telephone. “What a person … seeks to preserve as private, even in an area accessible to the public, may be constitutionally protected.” We have therefore repeatedly rejected attempts to justify a State’s invasion of privacy on the ground that the privacy is not absolute.

    Nor is it dispositive that “respondents placed their refuse at the curb for the express purpose of conveying it to a third party, … who might himself have sorted through respondents’ trash or permitted others, such as the police, to do so.” In the first place, Greenwood can hardly be faulted for leaving trash on his curb when a county ordinance commanded him to do so and prohibited him from disposing of it in any other way. Unlike in other circumstances where privacy is compromised, Greenwood could not “avoid exposing personal belongings … by simply leaving them at home.” More importantly, even the voluntary relinquishment of possession or control over an effect does not necessarily amount to a relinquishment of a privacy expectation in it. Were it otherwise, a letter or package would lose all Fourth Amendment protection when placed in a mailbox or other depository with the “express purpose” of entrusting it to the postal officer or a private carrier; those bailees are just as likely as trash collectors (and certainly have greater incentive) to “sor[t] through” the personal effects entrusted to them, “or permi[t] others, such as police to do so.” Yet, it has been clear for at least 110 years that the possibility of such an intrusion does not justify a warrantless search by police in the first instance.


    In holding that the warrantless search of Greenwood’s trash was consistent with the Fourth Amendment, the Court paints a grim picture of our society. It depicts a society in which local authorities may command their citizens to dispose of their personal effects in the manner least protective of the “sanctity of [the] home and the privacies of life,” and then monitor them arbitrarily and without judicial oversight—a society that is not prepared to recognize as reasonable an individual’s expectation of privacy in the most private of personal effects sealed in an opaque container and disposed of in a manner designed to commingle it imminently and inextricably with the trash of others. The American society with which I am familiar “chooses to dwell in reasonable security and freedom from surveillance,” and is more dedicated to individual liberty and more sensitive to intrusions on the sanctity of the home than the Court is willing to acknowledge.

    I dissent.

    Notes, Comments, and Questions

    The Greenwood Court determined there was no search because there was no objective expectation of privacy in trash placed by the curb; the narcotics evidence in that trash was readily accessible to the public, placed at the curb for conveyance to a third party, and police are not expected to “avert their eyes” from publicly observable criminal behavior. Should there be a different outcome if the illegality of the contents is not so readily observable?

    Consider the case of a tax preparer suspected of defrauding the government. The IRS agents collect his curbside trash for weeks only to discover the trash bags are filled with documents shredded into 5/32 inch strips. After reconstructing the documents (take a moment to consider the time and effort necessary to do so), the IRS has sufficient probable cause for a search warrant. Search or no search? Why or why not?

    Consider police who extract DNA evidence from curbside trash. Does a defendant have a reasonable expectation of privacy in his DNA evidence on cups, bottles, or condoms placed in the trash on the curb? Several recent murders have been solved using a combination genetic ancestry research (to narrow the suspect pool) and DNA from trash to zero in on the defendant. [ABC News]

    When interpreting its own law (such as a state constitution), a state court can recognize a “search” where courts applying the federal constitution would not. What are some arguments for and against states using different definitions of “search” than the Supreme Court has used when interpreting the Fourth Amendment? See State v. Morris, 680 A.2d 90, 92–93 (Vt 1996) (“One may accept the possibility that one’s garbage is susceptible to invasion by raccoons or other scavengers, and yet at the same time reasonably expect that the government will not systematically examine one’s trash bags in the hopes of finding evidence of criminal conduct.”).

    * * *

    In Katz, the Court decided that not all Fourth Amendment “searches” involve physical intrusion into an area in which someone enjoys a reasonable expectation of privacy. In the next case, the Court applies this principle to the use of thermal imaging technology.

    Supreme Court of the United States

    Danny Lee Kyllo v. United States

    Decided June 11, 2001—533 U.S. 27

    Justice SCALIA delivered the opinion of the Court.

    This case presents the question whether the use of a thermal-imaging device aimed at a private home from a public street to detect relative amounts of heat within the home constitutes a “search” within the meaning of the Fourth Amendment.


    In 1991 Agent William Elliott of the United States Department of the Interior came to suspect that marijuana was being grown in the home belonging to petitioner Danny Kyllo, part of a triplex on Rhododendron Drive in Florence, Oregon. Indoor marijuana growth typically requires high-intensity lamps. In order to determine whether an amount of heat was emanating from petitioner’s home consistent with the use of such lamps, at 3:20 a.m. on January 16, 1992, Agent Elliott and Dan Haas used an Agema Thermovision 210 thermal imager to scan the triplex. Thermal imagers detect infrared radiation, which virtually all objects emit but which is not visible to the naked eye. The imager converts radiation into images based on relative warmth—black is cool, white is hot, shades of gray connote relative differences; in that respect, it operates somewhat like a video camera showing heat images. The scan of Kyllo’s home took only a few minutes and was performed from the passenger seat of Agent Elliott’s vehicle across the street from the front of the house and also from the street in back of the house. The scan showed that the roof over the garage and a side wall of petitioner’s home were relatively hot compared to the rest of the home and substantially warmer than neighboring homes in the triplex. Agent Elliott concluded that petitioner was using halide lights to grow marijuana in his house, which indeed he was. Based on tips from informants, utility bills, and the thermal imaging, a Federal Magistrate Judge issued a warrant authorizing a search of petitioner’s home, and the agents found an indoor growing operation involving more than 100 plants. Petitioner was indicted on one count of manufacturing marijuana. He unsuccessfully moved to suppress the evidence seized from his home and then entered a conditional guilty plea.

    The Court of Appeals for the Ninth Circuit remanded the case for an evidentiary hearing regarding the intrusiveness of thermal imaging. On remand the District Court found that the Agema 210 “is a non-intrusive device which emits no rays or beams and shows a crude visual image of the heat being radiated from the outside of the house”; it “did not show any people or activity within the walls of the structure”; “[t]he device used cannot penetrate walls or windows to reveal conversations or human activities”; and “[n]o intimate details of the home were observed.” Based on these findings, the District Court upheld the validity of the warrant that relied in part upon the thermal imaging, and reaffirmed its denial of the motion to suppress. A divided Court of Appeals initially reversed, but that opinion was withdrawn and the panel (after a change in composition) affirmed, with Judge Noonan dissenting. The court held that petitioner had shown no subjective expectation of privacy because he had made no attempt to conceal the heat escaping from his home and even if he had, there was no objectively reasonable expectation of privacy because the imager “did not expose any intimate details of Kyllo’s life,” only “amorphous ‘hot spots’ on the roof and exterior wall.” We granted certiorari.


    “At the very core” of the Fourth Amendment “stands the right of a man to retreat into his own home and there be free from unreasonable governmental intrusion.” With few exceptions, the question whether a warrantless search of a home is reasonable and hence constitutional must be answered no.

    On the other hand, the antecedent question whether or not a Fourth Amendment “search” has occurred is not so simple under our precedent. The permissibility of ordinary visual surveillance of a home used to be clear because, well into the 20th century, our Fourth Amendment jurisprudence was tied to common-law trespass. Visual surveillance was unquestionably lawful because “‘the eye cannot by the laws of England be guilty of a trespass.’” We have since decoupled violation of a person’s Fourth Amendment rights from trespassory violation of his property, but the lawfulness of warrantless visual surveillance of a home has still been preserved. As we observed in California v. Ciraolo, 476 U.S. 207 (1986), “[t]he Fourth Amendment protection of the home has never been extended to require law enforcement officers to shield their eyes when passing by a home on public thoroughfares.”

    One might think that the new validating rationale would be that examining the portion of a house that is in plain public view, while it is a “search” despite the absence of trespass, is not an “unreasonable” one under the Fourth Amendment. But in fact we have held that visual observation is no “search” at all—perhaps in order to preserve somewhat more intact our doctrine that warrantless searches are presumptively unconstitutional. In assessing when a search is not a search, we have applied somewhat in reverse the principle first enunciated in Katz v. United States. As Justice Harlan’s oft-quoted concurrence described it, a Fourth Amendment search occurs when the government violates a subjective expectation of privacy that society recognizes as reasonable. We have subsequently applied this principle to hold that a Fourth Amendment search does not occur—even when the explicitly protected location of a house is concerned—unless “the individual manifested a subjective expectation of privacy in the object of the challenged search,” and “society [is] willing to recognize that expectation as reasonable.” We have applied this test in holding that it is not a search for the police to use a pen register at the phone company to determine what numbers were dialed in a private home, and we have applied the test on two different occasions in holding that aerial surveillance of private homes and surrounding areas does not constitute a search.

    The present case involves officers on a public street engaged in more than naked-eye surveillance of a home. We have previously reserved judgment as to how much technological enhancement of ordinary perception from such a vantage point, if any, is too much. While we upheld enhanced aerial photography of an industrial complex in Dow Chemical, we noted that we found “it important that this is not an area immediately adjacent to a private home, where privacy expectations are most heightened.” 476 U.S. 227 (1986).


    It would be foolish to contend that the degree of privacy secured to citizens by the Fourth Amendment has been entirely unaffected by the advance of technology. The question we confront today is what limits there are upon this power of technology to shrink the realm of guaranteed privacy.

    The Katz test—whether the individual has an expectation of privacy that society is prepared to recognize as reasonable—has often been criticized as circular, and hence subjective and unpredictable. While it may be difficult to refine Katz when the search of areas such as telephone booths, automobiles, or even the curtilage and uncovered portions of residences is at issue, in the case of the search of the interior of homes—the prototypical and hence most commonly litigated area of protected privacy—there is a ready criterion, with roots deep in the common law, of the minimal expectation of privacy that exists, and that is acknowledged to be reasonable. To withdraw protection of this minimum expectation would be to permit police technology to erode the privacy guaranteed by the Fourth Amendment. We think that obtaining by sense-enhancing technology any information regarding the interior of the home that could not otherwise have been obtained without physical “intrusion into a constitutionally protected area” constitutes a search—at least where (as here) the technology in question is not in general public use. This assures preservation of that degree of privacy against government that existed when the Fourth Amendment was adopted. On the basis of this criterion, the information obtained by the thermal imager in this case was the product of a search.

    The Government maintains, however, that the thermal imaging must be upheld because it detected “only heat radiating from the external surface of the house.” But just as a thermal imager captures only heat emanating from a house, so also a powerful directional microphone picks up only sound emanating from a house—and a satellite capable of scanning from many miles away would pick up only visible light emanating from a house. We rejected such a mechanical interpretation of the Fourth Amendment in Katz, where the eavesdropping device picked up only sound waves that reached the exterior of the phone booth. Reversing that approach would leave the homeowner at the mercy of advancing technology—including imaging technology that could discern all human activity in the home. While the technology used in the present case was relatively crude, the rule we adopt must take account of more sophisticated systems that are already in use or in development.

    The Government also contends that the thermal imaging was constitutional because it did not “detect private activities occurring in private areas.” The Fourth Amendment’s protection of the home has never been tied to measurement of the quality or quantity of information obtained. In the home, our cases show, all details are intimate details, because the entire area is held safe from prying government eyes.

    Limiting the prohibition of thermal imaging to “intimate details” would not only be wrong in principle; it would be impractical in application, failing to provide “a workable accommodation between the needs of law enforcement and the interests protected by the Fourth Amendment.” To begin with, there is no necessary connection between the sophistication of the surveillance equipment and the “intimacy” of the details that it observes—which means that one cannot say (and the police cannot be assured) that use of the relatively crude equipment at issue here will always be lawful. The Agema Thermovision 210 might disclose, for example, at what hour each night the lady of the house takes her daily sauna and bath—a detail that many would consider “intimate”; and a much more sophisticated system might detect nothing more intimate than the fact that someone left a closet light on. We could not, in other words, develop a rule approving only that through-the-wall surveillance which identifies objects no smaller than 36 by 36 inches, but would have to develop a jurisprudence specifying which home activities are “intimate” and which are not. And even when (if ever) that jurisprudence were fully developed, no police officer would be able to know in advance whether his through-the-wall surveillance picks up “intimate” details—and thus would be unable to know in advance whether it is constitutional.

    We have said that the Fourth Amendment draws “a firm line at the entrance to the house.” That line, we think, must be not only firm but also bright—which requires clear specification of those methods of surveillance that require a warrant. While it is certainly possible to conclude from the videotape of the thermal imaging that occurred in this case that no “significant” compromise of the homeowner’s privacy has occurred, we must take the long view, from the original meaning of the Fourth Amendment forward.

    Where, as here, the Government uses a device that is not in general public use, to explore details of the home that would previously have been unknowable without physical intrusion, the surveillance is a “search” and is presumptively unreasonable without a warrant.

    Since we hold the Thermovision imaging to have been an unlawful search, it will remain for the District Court to determine whether, without the evidence it provided, the search warrant issued in this case was supported by probable cause—and if not, whether there is any other basis for supporting admission of the evidence that the search pursuant to the warrant produced.

    The judgment of the Court of Appeals is reversed; the case is remanded for further proceedings consistent with this opinion.

    Justice STEVENS, with whom THE CHIEF JUSTICE, Justice O’CONNOR, and Justice KENNEDY join, dissenting.

    There is, in my judgment, a distinction of constitutional magnitude between “through-the-wall surveillance” that gives the observer or listener direct access to information in a private area, on the one hand, and the thought processes used to draw inferences from information in the public domain, on the other hand. The Court has crafted a rule that purports to deal with direct observations of the inside of the home, but the case before us merely involves indirect deductions from “off-the-wall” surveillance, that is, observations of the exterior of the home. Those observations were made with a fairly primitive thermal imager that gathered data exposed on the outside of petitioner’s home but did not invade any constitutionally protected interest in privacy. Moreover, I believe that the supposedly “bright-line” rule the Court has created in response to its concerns about future technological developments is unnecessary, unwise, and inconsistent with the Fourth Amendment.


    There is no need for the Court to craft a new rule to decide this case, as it is controlled by established principles from our Fourth Amendment jurisprudence. One of those core principles, of course, is that “searches and seizures inside a home without a warrant are presumptively unreasonable.” But it is equally well settled that searches and seizures of property in plain view are presumptively reasonable. “‘What a person knowingly exposes to the public, even in his own home or office, is not a subject of Fourth Amendment protection.’” That is the principle implicated here.

    While the Court “take[s] the long view” and decides this case based largely on the potential of yet-to-be-developed technology that might allow “through-the-wall surveillance,” this case involves nothing more than off-the-wall surveillance by law enforcement officers to gather information exposed to the general public from the outside of petitioner’s home. All that the infrared camera did in this case was passively measure heat emitted from the exterior surfaces of petitioner’s home; all that those measurements showed were relative differences in emission levels, vaguely indicating that some areas of the roof and outside walls were warmer than others. As still images from the infrared scans show, no details regarding the interior of petitioner’s home were revealed. Unlike an x-ray scan, or other possible “through-the-wall” techniques, the detection of infrared radiation emanating from the home did not accomplish “an unauthorized physical penetration into the premises,” nor did it “obtain information that it could not have obtained by observation from outside the curtilage of the house.”

    Indeed, the ordinary use of the senses might enable a neighbor or passerby to notice the heat emanating from a building, particularly if it is vented, as was the case here. Additionally, any member of the public might notice that one part of a house is warmer than another part or a nearby building if, for example, rainwater evaporates or snow melts at different rates across its surfaces. Such use of the senses would not convert into an unreasonable search if, instead, an adjoining neighbor allowed an officer onto her property to verify her perceptions with a sensitive thermometer. Nor, in my view, does such observation become an unreasonable search if made from a distance with the aid of a device that merely discloses that the exterior of one house, or one area of the house, is much warmer than another. Nothing more occurred in this case.

    Thus, the notion that heat emissions from the outside of a dwelling are a private matter implicating the protections of the Fourth Amendment (the text of which guarantees the right of people “to be secure in their … houses” against unreasonable searches and seizures (emphasis added)) is not only unprecedented but also quite difficult to take seriously. Heat waves, like aromas that are generated in a kitchen, or in a laboratory or opium den, enter the public domain if and when they leave a building. A subjective expectation that they would remain private is not only implausible but also surely not “one that society is prepared to recognize as ‘reasonable.’”

    To be sure, the homeowner has a reasonable expectation of privacy concerning what takes place within the home, and the Fourth Amendment’s protection against physical invasions of the home should apply to their functional equivalent. But the equipment in this case did not penetrate the walls of petitioner’s home, and while it did pick up “details of the home” that were exposed to the public, it did not obtain “any information regarding the interior of the home.” In the Court’s own words, based on what the thermal imager “showed” regarding the outside of petitioner’s home, the officers “concluded” that petitioner was engaging in illegal activity inside the home. It would be quite absurd to characterize their thought processes as “searches,” regardless of whether they inferred (rightly) that petitioner was growing marijuana in his house, or (wrongly) that “the lady of the house [was taking] her daily sauna and bath.” In either case, the only conclusions the officers reached concerning the interior of the home were at least as indirect as those that might have been inferred from the contents of discarded garbage, or, as in this case, subpoenaed utility records. For the first time in its history, the Court assumes that an inference can amount to a Fourth Amendment violation.

    Notwithstanding the implications of today’s decision, there is a strong public interest in avoiding constitutional litigation over the monitoring of emissions from homes, and over the inferences drawn from such monitoring. Just as “the police cannot reasonably be expected to avert their eyes from evidence of criminal activity that could have been observed by any member of the public,” so too public officials should not have to avert their senses or their equipment from detecting emissions in the public domain such as excessive heat, traces of smoke, suspicious odors, odorless gases, airborne particulates, or radioactive emissions, any of which could identify hazards to the community. In my judgment, monitoring such emissions with “sense-enhancing technology,” and drawing useful conclusions from such monitoring, is an entirely reasonable public service.

    On the other hand, the countervailing privacy interest is at best trivial. After all, homes generally are insulated to keep heat in, rather than to prevent the detection of heat going out, and it does not seem to me that society will suffer from a rule requiring the rare homeowner who both intends to engage in uncommon activities that produce extraordinary amounts of heat, and wishes to conceal that production from outsiders, to make sure that the surrounding area is well insulated. The interest in concealing the heat escaping from one’s house pales in significance to “the chief evil against which the wording of the Fourth Amendment is directed,” the “physical entry of the home.”

    Since what was involved in this case was nothing more than drawing inferences from off-the-wall surveillance, rather than any “through-the-wall” surveillance, the officers’ conduct did not amount to a search and was perfectly reasonable.


    Instead of trying to answer the question whether the use of the thermal imager in this case was even arguably unreasonable, the Court has fashioned a rule that is intended to provide essential guidance for the day when “more sophisticated systems” gain the “ability to ‘see’ through walls and other opaque barriers.” The newly minted rule encompasses “obtaining [1] by sense-enhancing technology [2] any information regarding the interior of the home [3] that could not otherwise have been obtained without physical intrusion into a constitutionally protected area … [4] at least where (as here) the technology in question is not in general public use.” In my judgment, the Court’s new rule is at once too broad and too narrow, and is not justified by the Court’s explanation for its adoption. As I have suggested, I would not erect a constitutional impediment to the use of sense-enhancing technology unless it provides its user with the functional equivalent of actual presence in the area being searched.

    Despite the Court’s attempt to draw a line that is “not only firm but also bright,” the contours of its new rule are uncertain because its protection apparently dissipates as soon as the relevant technology is “in general public use.” Yet how much use is general public use is not even hinted at by the Court’s opinion, which makes the somewhat doubtful assumption that the thermal imager used in this case does not satisfy that criterion. In any event, putting aside its lack of clarity, this criterion is somewhat perverse because it seems likely that the threat to privacy will grow, rather than recede, as the use of intrusive equipment becomes more readily available.

    The application of the Court’s new rule to “any information regarding the interior of the home,” is unnecessarily broad. If it takes sensitive equipment to detect an odor that identifies criminal conduct and nothing else, the fact that the odor emanates from the interior of a home should not provide it with constitutional protection. The criterion, moreover, is too sweeping in that information “regarding” the interior of a home apparently is not just information obtained through its walls, but also information concerning the outside of the building that could lead to (however many) inferences “regarding” what might be inside. Under that expansive view, I suppose, an officer using an infrared camera to observe a man silently entering the side door of a house at night carrying a pizza might conclude that its interior is now occupied by someone who likes pizza, and by doing so the officer would be guilty of conducting an unconstitutional “search” of the home.

    Because the new rule applies to information regarding the “interior” of the home, it is too narrow as well as too broad. Clearly, a rule that is designed to protect individuals from the overly intrusive use of sense-enhancing equipment should not be limited to a home. If such equipment did provide its user with the functional equivalent of access to a private place—such as, for example, the telephone booth involved in Katz, or an office building—then the rule should apply to such an area as well as to a home.

    The final requirement of the Court’s new rule, that the information “could not otherwise have been obtained without physical intrusion into a constitutionally protected area,” also extends too far as the Court applies it. As noted, the Court effectively treats the mental process of analyzing data obtained from external sources as the equivalent of a physical intrusion into the home. As I have explained, however, the process of drawing inferences from data in the public domain should not be characterized as a search.


    Although the Court is properly and commendably concerned about the threats to privacy that may flow from advances in the technology available to the law enforcement profession, it has unfortunately failed to heed the tried and true counsel of judicial restraint. Instead of concentrating on the rather mundane issue that is actually presented by the case before it, the Court has endeavored to craft an all-encompassing rule for the future. It would be far wiser to give legislators an unimpeded opportunity to grapple with these emerging issues rather than to shackle them with prematurely devised constitutional constraints.

    I respectfully dissent.

    Notes, Comments, and Questions

    The Kyllo majority reasoned in 2001 (in a case about police conduct that occurred in 1991) that the use of thermal imaging constituted a search because the technology was “not in general public use.”

    Today, however, the general public has many uses for thermal imaging, from HVAC performance testing to hunting to wildlife rescue to evaluating the performance of kitchen devices.

    Agema Infrared Systems, the Swedish corporation that manufactured the “Agema Thermovision 210” at issue in Kyllo, was acquired by FLIR Systems Inc. in 1998. Headquartered in Oregon, FLIR now sells a $200 thermal imaging camera (the “FLIR ONE”) that can attach to a smartphone, with fancier versions available for higher prices. According to the FLIR product page, one can use the FLIR ONE to “[f]ind problems around the home fast, like where you’re losing heat, how your insulation’s holding up, electrical problems, and water damage – all of which are point-and-shoot easy to find.” It also suggests, “See in the dark and explore the natural world safely with the FLIR ONE. Watch animals in their natural habitat and even use it to find your lost pet … or what they might have left behind in the yard.” Another suggested use from the advertisement: “Detecting tiny variations in heat means that you can see in total darkness, create new kinds of art, and discover new things about your world every day… or help your child with their science fair experiment.”

    Consider a police officer who uses such a device to investigate a suspected drug-grower’s home. He sees images consistent with growing drugs. He shows the images to a judge, who grants a search warrant. Officers find drugs in the house, and prosecutors have charged the owner with drug crimes. Search or no search? Why or why not?

    In January 2020, the City Counsel of Bessemer, Michigan voted to purchase “an odor-detecting device as a means of addressing growing complaints about marijuana odor.” The device is called a “Nasal Ranger,” and the company that sells it describes it as “the ‘state-of-the-art’ in field olfactometry for confidently measuring and quantifying odor strength in the ambient air.” According to St. Croix Sensory, Inc., “The portable Nasal Ranger Field Olfactometer determines ambient odor Dilution-to-Threshold (D/T) concentration objectively with your trained nose.” If a Bessmer police officer stands on a public sidewalk and uses the Nasal Ranger to detect marijuana odors emanating from a house, is that a search? Why or why not?

    * * *

    In his majority opinion in United States v. Jones, Justice Scalia distinguished the facts before the Court in that case from those of a previous case—involving a “beeper”—upon which the government attempted to rely in its effort to justify placing a GPS device on a vehicle. Here is the “beeper” case.

    Supreme Court of the United States

    United States v. Leroy Carlton Knotts

    Decided March 2, 1983—460 U.S. 276

    REHNQUIST, Justice.

    A beeper is a radio transmitter, usually battery operated, which emits periodic signals that can be picked up by a radio receiver. In this case, a beeper was placed in a five gallon drum containing chloroform purchased by one of respondent’s codefendants. By monitoring the progress of a car carrying the chloroform Minnesota law enforcement agents were able to trace the can of chloroform from its place of purchase in Minneapolis, Minnesota to respondent’s secluded cabin near Shell Lake, Wisconsin. The issue presented by the case is whether such use of a beeper violated respondent’s rights secured by the Fourth Amendment to the United States Constitution.


    Respondent and two codefendants were charged in the United States District Court for the District of Minnesota with conspiracy to manufacture controlled substances, including but not limited to methamphetamine.

    Suspicion attached to this trio when the 3M Company, which manufactures chemicals in St. Paul, notified a narcotics investigator for the Minnesota Bureau of Criminal Apprehension that Armstrong, a former 3M employee, had been stealing chemicals which could be used in manufacturing illicit drugs. Visual surveillance of Armstrong revealed that after leaving the employ of 3M Company, he had been purchasing similar chemicals from the Hawkins Chemical Company in Minneapolis. The Minnesota narcotics officers observed that after Armstrong had made a purchase, he would deliver the chemicals to codefendant Petschen.

    With the consent of the Hawkins Chemical Company, officers installed a beeper inside a five gallon container of chloroform, one of the so-called “precursor” chemicals used to manufacture illicit drugs. Hawkins agreed that when Armstrong next purchased chloroform, the chloroform would be placed in this particular container. When Armstrong made the purchase, officers followed the car in which the chloroform had been placed, maintaining contact by using both visual surveillance and a monitor which received the signals sent from the beeper.

    Armstrong proceeded to Petschen’s house, where the container was transferred to Petschen’s automobile. Officers then followed that vehicle eastward towards the state line, across the St. Croix River, and into Wisconsin. During the latter part of this journey, Petschen began making evasive maneuvers, and the pursuing agents ended their visual surveillance. At about the same time officers lost the signal from the beeper, but with the assistance of a monitoring device located in a helicopter the approximate location of the signal was picked up again about one hour later. The signal now was stationary and the location identified was a cabin occupied by respondent near Shell Lake, Wisconsin. The record before us does not reveal that the beeper was used after the location in the area of the cabin had been initially determined.

    Relying on the location of the chloroform derived through the use of the beeper and additional information obtained during three days of intermittent visual surveillance of respondent’s cabin, officers secured a search warrant. During execution of the warrant, officers discovered a fully operable, clandestine drug laboratory in the cabin. In the laboratory area officers found formulas for amphetamine and methamphetamine, over $10,000 worth of laboratory equipment, and chemicals in quantities sufficient to produce 14 pounds of pure amphetamine. Under a barrel outside the cabin, officers located the five gallon container of chloroform.

    After his motion to suppress evidence based on the warrantless monitoring of the beeper was denied, respondent was convicted for conspiring to manufacture controlled substances. He was sentenced to five years imprisonment. A divided panel of the United States Court of Appeals for the Eighth Circuit reversed the conviction, finding that the monitoring of the beeper was prohibited by the Fourth Amendment because its use had violated respondent’s reasonable expectation of privacy, and that all information derived after the location of the cabin was a fruit of the illegal beeper monitoring. We granted certiorari, and we now reverse the judgment of the Court of Appeals.


    The governmental surveillance conducted by means of the beeper in this case amounted principally to the following of an automobile on public streets and highways. We have commented more than once on the diminished expectation of privacy in an automobile:

    “One has a lesser expectation of privacy in a motor vehicle because its function is transportation and it seldom serves as one’s residence or as the repository of personal effects. A car has little capacity for escaping public scrutiny. It travels public thoroughfares where both its occupants and its contents are in plain view.”

    A person travelling in an automobile on public thoroughfares has no reasonable expectation of privacy in his movements from one place to another. When Petschen travelled over the public streets he voluntarily conveyed to anyone who wanted to look the fact that he was travelling over particular roads in a particular direction, the fact of whatever stops he made, and the fact of his final destination when he exited from public roads onto private property.

    Respondent Knotts, as the owner of the cabin and surrounding premises to which Petschen drove, undoubtedly had the traditional expectation of privacy within a dwelling place insofar as the cabin was concerned:

    “Crime, even in the privacy of one’s own quarters, is, of course, of grave concern to society, and the law allows such crime to be reached on proper showing. The right of officers to thrust themselves into a home is also of grave concern, not only to the individual, but to a society which chooses to dwell in reasonable security and freedom from surveillance. When the right of privacy must reasonably yield to the right of search is, as a rule, to be decided by a judicial officer, not by a policeman or government enforcement agent.”

    But no such expectation of privacy extended to the visual observation of Petschen’s automobile arriving on his premises after leaving a public highway, nor to movements of objects such as the drum of chloroform outside the cabin in the “open fields.”

    Visual surveillance from public places along Petschen’s route or adjoining Knotts’ premises would have sufficed to reveal all of these facts to the police. The fact that the officers in this case relied not only on visual surveillance, but on the use of the beeper to signal the presence of Petschen’s automobile to the police receiver, does not alter the situation. Nothing in the Fourth Amendment prohibited the police from augmenting the sensory faculties bestowed upon them at birth with such enhancement as science and technology afforded them in this case.

    Respondent specifically attacks the use of the beeper insofar as it was used to determine that the can of chloroform had come to rest on his property at Shell Lake, Wisconsin. He repeatedly challenges the “use of the beeper to determine the location of the chemical drum at Respondent’s premises[;]” he states that “[t]he government thus overlooks the fact that this case involves the sanctity of Respondent’s residence, which is accorded the greatest protection available under the Fourth Amendment.”

    We think that respondent’s contentions to some extent lose sight of the limited use which the government made of the signals from this particular beeper. As we have noted, nothing in this record indicates that the beeper signal was received or relied upon after it had indicated that the drum containing the chloroform had ended its automotive journey at rest on respondent’s premises in rural Wisconsin. Admittedly, because of the failure of the visual surveillance, the beeper enabled the law enforcement officials in this case to ascertain the ultimate resting place of the chloroform when they would not have been able to do so had they relied solely on their naked eyes. But scientific enhancement of this sort raises no constitutional issues which visual surveillance would not also raise. A police car following Petschen at a distance throughout his journey could have observed him leaving the public highway and arriving at the cabin owned by respondent, with the drum of chloroform still in the car. This fact, along with others, was used by the government in obtaining a search warrant which led to the discovery of the clandestine drug laboratory. But there is no indication that the beeper was used in any way to reveal information as to the movement of the drum within the cabin, or in any way that would not have been visible to the naked eye from outside the cabin.

    We thus return to the question posed at the beginning of our inquiry in discussing Katz, supra; did monitoring the beeper signals complained of by respondent invade any legitimate expectation of privacy on his part? For the reasons previously stated, we hold they did not. Since they did not, there was neither a “search” nor a “seizure” within the contemplation of the Fourth Amendment. The judgment of the Court of Appeals is therefore


    Justice STEVENS, with whom Justice BRENNAN, and Justice MARSHALL join, concurring in the judgment.

    Since the respondent in this case has never questioned the installation of the radio transmitter in the chloroform drum, I agree that it was entirely reasonable for the police officers to make use of the information received over the airwaves when they were trying to ascertain the ultimate destination of the chloroform. I do not join the Court’s opinion, however, because it contains two unnecessarily broad dicta: one distorts the record in this case, and both may prove confusing to courts that must apply this decision in the future.

    First, the Court implies that the chloroform drum was parading in “open fields” outside of the cabin, in a manner tantamount to its public display on the highways. The record does not support that implication.

    Second, the Court suggests that the Fourth Amendment does not inhibit “the police from augmenting the sensory facilities bestowed upon them at birth with such enhancement as science and technology afforded them.” But the Court held to the contrary in Katz v. United States. Although the augmentation in this case was unobjectionable, it by no means follows that the use of electronic detection techniques does not implicate especially sensitive concerns.

    Accordingly, I concur in the judgment.

    * * *

    The ubiquitous use of mobile phones, by which users not only have conversations but also transmit all sorts of sensitive data, has raised important questions about when the government may intercept information transmitted by phone users. This is not, however, a new issue. More than four decades ago, police obtained certain information from a suspect’s telephone company, and prosecutors used that information against the defendant at trial. Here is the resulting Fourth Amendment case.

    Supreme Court of the United States

    Michael Lee Smith v. Maryland

    Decided June 20, 1979—442 U.S. 735

    Mr. Justice BLACKMUN delivered the opinion of the Court.

    This case presents the question whether the installation and use of a pen register1 constitutes a “search” within the meaning of the Fourth Amendment.


    On March 5, 1976, in Baltimore, Md., Patricia McDonough was robbed. She gave the police a description of the robber and of a 1975 Monte Carlo automobile she had observed near the scene of the crime. After the robbery, McDonough began receiving threatening and obscene phone calls from a man identifying himself as the robber. On one occasion, the caller asked that she step out on her front porch; she did so, and saw the 1975 Monte Carlo she had earlier described to police moving slowly past her home. On March 16, police spotted a man who met McDonough’s description driving a 1975 Monte Carlo in her neighborhood. By tracing the license plate number, police learned that the car was registered in the name of petitioner, Michael Lee Smith.

    The next day, the telephone company, at police request, installed a pen register at its central offices to record the numbers dialed from the telephone at petitioner’s home. The police did not get a warrant or court order before having the pen register installed. The register revealed that on March 17 a call was placed from petitioner’s home to McDonough’s phone. On the basis of this and other evidence, the police obtained a warrant to search petitioner’s residence. The search revealed that a page in petitioner’s phone book was turned down to the name and number of Patricia McDonough; the phone book was seized. Petitioner was arrested, and a six-man lineup was held on March 19. McDonough identified petitioner as the man who had robbed her.

    Petitioner was indicted in the Criminal Court of Baltimore for robbery. By pretrial motion, he sought to suppress “all fruits derived from the pen register” on the ground that the police had failed to secure a warrant prior to its installation. The trial court denied the suppression motion, holding that the warrantless installation of the pen register did not violate the Fourth Amendment. Petitioner then waived a jury, and the case was submitted to the court on an agreed statement of facts. The pen register tape (evidencing the fact that a phone call had been made from petitioner’s phone to McDonough’s phone) and the phone book seized in the search of petitioner’s residence were admitted into evidence against him. Petitioner was convicted, and was sentenced to six years. He appealed to the Maryland Court of Special Appeals, but the Court of Appeals of Maryland issued a writ of certiorari to the intermediate court in advance of its decision in order to consider whether the pen register evidence had been properly admitted at petitioner’s trial.

    The Court of Appeals affirmed the judgment of conviction, holding that “there is no constitutionally protected reasonable expectation of privacy in the numbers dialed into a telephone system and hence no search within the fourth amendment is implicated by the use of a pen register installed at the central offices of the telephone company.” Because there was no “search,” the court concluded, no warrant was needed. Certiorari was granted.


    In determining whether a particular form of government-initiated electronic surveillance is a “search” within the meaning of the Fourth Amendment, our lodestar is Katz v. United States, 389 U.S. 347 (1967).

    Consistently with Katz, this Court uniformly has held that the application of the Fourth Amendment depends on whether the person invoking its protection can claim a “justifiable,” a “reasonable,” or a “legitimate expectation of privacy” that has been invaded by government action. This inquiry, as Mr. Justice Harlan aptly noted in his Katz concurrence, normally embraces two discrete questions. The first is whether the individual, by his conduct, has “exhibited an actual (subjective) expectation of privacy”—whether, in the words of the Katz majority, the individual has shown that “he seeks to preserve [something] as private.” The second question is whether the individual’s subjective expectation of privacy is “one that society is prepared to recognize as ‘reasonable’”—whether, in the words of the Katz majority, the individual’s expectation, viewed objectively, is “justifiable” under the circumstances.


    In applying the Katz analysis to this case, it is important to begin by specifying precisely the nature of the state activity that is challenged. The activity here took the form of installing and using a pen register. Since the pen register was installed on telephone company property at the telephone company’s central offices, petitioner obviously cannot claim that his “property” was invaded or that police intruded into a “constitutionally protected area.” Petitioner’s claim, rather, is that, notwithstanding the absence of a trespass, the State, as did the Government in Katz, infringed a “legitimate expectation of privacy” that petitioner held. Yet a pen register differs significantly from the listening device employed in Katz, for pen registers do not acquire the contents of communications. This Court recently noted:

    “Indeed, a law enforcement official could not even determine from the use of a pen register whether a communication existed. These devices do not hear sound. They disclose only the telephone numbers that have been dialed—a means of establishing communication. Neither the purport of any communication between the caller and the recipient of the call, their identities, nor whether the call was even completed is disclosed by pen registers.”

    Given a pen register’s limited capabilities, therefore, petitioner’s argument that its installation and use constituted a “search” necessarily rests upon a claim that he had a “legitimate expectation of privacy” regarding the numbers he dialed on his phone.

    This claim must be rejected. First, we doubt that people in general entertain any actual expectation of privacy in the numbers they dial. All telephone users realize that they must “convey” phone numbers to the telephone company, since it is through telephone company switching equipment that their calls are completed. All subscribers realize, moreover, that the phone company has facilities for making permanent records of the numbers they dial, for they see a list of their long-distance (toll) calls on their monthly bills. In fact, pen registers and similar devices are routinely used by telephone companies “for the purposes of checking billing operations, detecting fraud and preventing violations of law.” Electronic equipment is used not only to keep billing records of toll calls, but also “to keep a record of all calls dialed from a telephone which is subject to a special rate structure.” Pen registers are regularly employed “to determine whether a home phone is being used to conduct a business, to check for a defective dial, or to check for overbilling.” Although most people may be oblivious to a pen register’s esoteric functions, they presumably have some awareness of one common use: to aid in the identification of persons making annoying or obscene calls. Most phone books tell subscribers, on a page entitled “Consumer Information,” that the company “can frequently help in identifying to the authorities the origin of unwelcome and troublesome calls.” Telephone users, in sum, typically know that they must convey numerical information to the phone company; that the phone company has facilities for recording this information; and that the phone company does in fact record this information for a variety of legitimate business purposes. Although subjective expectations cannot be scientifically gauged, it is too much to believe that telephone subscribers, under these circumstances, harbor any general expectation that the numbers they dial will remain secret.

    Petitioner argues, however, that, whatever the expectations of telephone users in general, he demonstrated an expectation of privacy by his own conduct here, since he “us[ed] the telephone in his house to the exclusion of all others.” But the site of the call is immaterial for purposes of analysis in this case. Although petitioner’s conduct may have been calculated to keep the contents of his conversation private, his conduct was not and could not have been calculated to preserve the privacy of the number he dialed. Regardless of his location, petitioner had to convey that number to the telephone company in precisely the same way if he wished to complete his call. The fact that he dialed the number on his home phone rather than on some other phone could make no conceivable difference, nor could any subscriber rationally think that it would.

    Second, even if petitioner did harbor some subjective expectation that the phone numbers he dialed would remain private, this expectation is not “one that society is prepared to recognize as ‘reasonable.’” This Court consistently has held that a person has no legitimate expectation of privacy in information he voluntarily turns over to third parties.

    This analysis dictates that petitioner can claim no legitimate expectation of privacy here. When he used his phone, petitioner voluntarily conveyed numerical information to the telephone company and “exposed” that information to its equipment in the ordinary course of business. In so doing, petitioner assumed the risk that the company would reveal to police the numbers he dialed. The switching equipment that processed those numbers is merely the modern counterpart of the operator who, in an earlier day, personally completed calls for the subscriber. Petitioner concedes that if he had placed his calls through an operator, he could claim no legitimate expectation of privacy. We are not inclined to hold that a different constitutional result is required because the telephone company has decided to automate.

    We therefore conclude that petitioner in all probability entertained no actual expectation of privacy in the phone numbers he dialed, and that, even if he did, his expectation was not “legitimate.” The installation and use of a pen register, consequently, was not a “search,” and no warrant was required. The judgment of the Maryland Court of Appeals is affirmed.

    Mr. Justice POWELL took no part in the consideration or decision of this case.

    Mr. Justice STEWART, with whom Mr. Justice BRENNAN joins, dissenting.

    I am not persuaded that the numbers dialed from a private telephone fall outside the constitutional protection of the Fourth and Fourteenth Amendments.

    In Katz v. United States, the Court acknowledged the “vital role that the public telephone has come to play in private communication[s].” The role played by a private telephone is even more vital, and since Katz it has been abundantly clear that telephone conversations carried on by people in their homes or offices are fully protected.

    Nevertheless, the Court today says that those safeguards do not extend to the numbers dialed from a private telephone, apparently because when a caller dials a number the digits may be recorded by the telephone company for billing purposes. But that observation no more than describes the basic nature of telephone calls. A telephone call simply cannot be made without the use of telephone company property and without payment to the company for the service. The telephone conversation itself must be electronically transmitted by telephone company equipment, and may be recorded or overheard by the use of other company equipment. Yet we have squarely held that the user of even a public telephone is entitled “to assume that the words he utters into the mouthpiece will not be broadcast to the world.”

    The central question in this case is whether a person who makes telephone calls from his home is entitled to make a similar assumption about the numbers he dials. What the telephone company does or might do with those numbers is no more relevant to this inquiry than it would be in a case involving the conversation itself. It is simply not enough to say, after Katz, that there is no legitimate expectation of privacy in the numbers dialed because the caller assumes the risk that the telephone company will disclose them to the police.

    I think that the numbers dialed from a private telephone—like the conversations that occur during a call—are within the constitutional protection recognized in Katz. It seems clear to me that information obtained by pen register surveillance of a private telephone is information in which the telephone subscriber has a legitimate expectation of privacy. The information captured by such surveillance emanates from private conduct within a person’s home or office—locations that without question are entitled to Fourth and Fourteenth Amendment protection. Further, that information is an integral part of the telephonic communication that under Katz is entitled to constitutional protection, whether or not it is captured by a trespass into such an area.

    The numbers dialed from a private telephone—although certainly more prosaic than the conversation itself—are not without “content.” Most private telephone subscribers may have their own numbers listed in a publicly distributed directory, but I doubt there are any who would be happy to have broadcast to the world a list of the local or long distance numbers they have called. This is not because such a list might in some sense be incriminating, but because it easily could reveal the identities of the persons and the places called, and thus reveal the most intimate details of a person’s life.

    I respectfully dissent.

    Mr. Justice MARSHALL, with whom Mr. Justice BRENNAN joins, dissenting.

    The Court concludes that because individuals have no actual or legitimate expectation of privacy in information they voluntarily relinquish to telephone companies, the use of pen registers by government agents is immune from Fourth Amendment scrutiny. I respectfully dissent.

    Applying the standards set forth in Katz v. United States, the Court first determines that telephone subscribers have no subjective expectations of privacy concerning the numbers they dial. To reach this conclusion, the Court posits that individuals somehow infer from the long-distance listings on their phone bills, and from the cryptic assurances of “help” in tracing obscene calls included in “most” phone books, that pen registers are regularly used for recording local calls. But even assuming, as I do not, that individuals “typically know” that a phone company monitors calls for internal reasons, it does not follow that they expect this information to be made available to the public in general or the government in particular. Privacy is not a discrete commodity, possessed absolutely or not at all. Those who disclose certain facts to a bank or phone company for a limited business purpose need not assume that this information will be released to other persons for other purposes.

    The crux of the Court’s holding, however, is that whatever expectation of privacy petitioner may in fact have entertained regarding his calls, it is not one “society is prepared to recognize as ‘reasonable’.” In so ruling, the Court determines that individuals who convey information to third parties have “assumed the risk” of disclosure to the government. This analysis is misconceived in two critical respects.

    Implicit in the concept of assumption of risk is some notion of choice. At least in the third-party consensual surveillance cases, which first incorporated risk analysis into Fourth Amendment doctrine, the defendant presumably had exercised some discretion in deciding who should enjoy his confidential communications. By contrast here, unless a person is prepared to forgo use of what for many has become a personal or professional necessity, he cannot help but accept the risk of surveillance. It is idle to speak of “assuming” risks in contexts where, as a practical matter, individuals have no realistic alternative.

    More fundamentally, to make risk analysis dispositive in assessing the reasonableness of privacy expectations would allow the government to define the scope of Fourth Amendment protections. For example, law enforcement officials, simply by announcing their intent to monitor the content of random samples of first-class mail or private phone conversations, could put the public on notice of the risks they would thereafter assume in such communications. Yet, although acknowledging this implication of its analysis, the Court is willing to concede only that, in some circumstances, a further “normative inquiry would be proper.” No meaningful effort is made to explain what those circumstances might be, or why this case is not among them.

    In my view, whether privacy expectations are legitimate within the meaning of Katz depends not on the risks an individual can be presumed to accept when imparting information to third parties, but on the risks he should be forced to assume in a free and open society. By its terms, the constitutional prohibition of unreasonable searches and seizures assigns to the judiciary some prescriptive responsibility. As Mr. Justice Harlan, who formulated the standard the Court applies today, himself recognized: “[s]ince it is the task of the law to form and project, as well as mirror and reflect, we should not … merely recite … risks without examining the desirability of saddling them upon society.” In making this assessment, courts must evaluate the “intrinsic character” of investigative practices with reference to the basic values underlying the Fourth Amendment. And for those “extensive intrusions that significantly jeopardize [individuals’] sense of security … more than self-restraint by law enforcement officials is required.”

    The use of pen registers, I believe, constitutes such an extensive intrusion. To hold otherwise ignores the vital role telephonic communication plays in our personal and professional relationships, as well as the First and Fourth Amendment interests implicated by unfettered official surveillance. Privacy in placing calls is of value not only to those engaged in criminal activity. The prospect of unregulated governmental monitoring will undoubtedly prove disturbing even to those with nothing illicit to hide. Many individuals, including members of unpopular political organizations or journalists with confidential sources, may legitimately wish to avoid disclosure of their personal contacts. Permitting governmental access to telephone records on less than probable cause may thus impede certain forms of political affiliation and journalistic endeavor that are the hallmark of a truly free society. Particularly given the Government’s previous reliance on warrantless telephonic surveillance to trace reporters’ sources and monitor protected political activity, I am unwilling to insulate use of pen registers from independent judicial review.

    Just as one who enters a public telephone booth is “entitled to assume that the words he utters into the mouthpiece will not be broadcast to the world,” so too, he should be entitled to assume that the numbers he dials in the privacy of his home will be recorded, if at all, solely for the phone company’s business purposes. Accordingly, I would require law enforcement officials to obtain a warrant before they enlist telephone companies to secure information otherwise beyond the government’s reach.

    Notes, Comments, and Questions

    For nearly forty years, the third-party disclosure doctrine stood as a broad general rule. The advance of technology, however, has raised questions about the doctrine. Consider, for example, the sort of data commonly transmitted by mobile phones. A person using GPS mapping on a phone is transmitting her location to a third party. Consider too the use of email, whether on a phone or on a computer. When someone sends email, the sender knows (at least at some basic level) that the contents of the message are transmitted to a third party before reaching the intended recipient. (Indeed, many third parties are likely involved.)

    Does a person who sends email despite knowing that messages travel via third-party servers (for example, on Gmail) forfeit any reasonable expectation of privacy with respect to the content of the messages?

    Imagine that a phone company routinely retains data concerning the locations of customers’ mobile phones. If police contact a phone company and obtain the location data for a particular customer’s phone from the previous month, is that a “search” or not? (We will confront this issue again in Chapter 5. Jot down your answer now, along with your reasoning, so that you can compare it with the reasoning used by the Court.)

    * * *

    In the next chapter, we will study the concept of “open fields,” to which the majority and dissent referred in Knotts. We will also consider police use of aerial surveillance, which required further elaboration of the Court’s definition of “search.”

    Then, in Chapter 5, during which we will wrap up our discussion of “what is a search,” we will consider (1) more recent judicial analysis inspired by modern phone technology and (2) police use of dogs.