The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.
The Court later adopted a new approach.
The premise that property interests control the right of the government to search and seize has been discredited. . . . We have recognized that the principal object of the Fourth Amendment is the protection of privacy rather than property, and have increasingly discarded fictional and procedural barriers rested on property concepts. 1 Thus, because the Amendment
protects people, not places, the requirement of actual physical trespass is dispensed with and electronic surveillance was made subject to the Amendment’s requirements. 2
The new test, propounded in Katz v. United States, is whether there is an expectation of privacy upon which one may
justifiably rely. 3
What a person knowingly exposes to the public, even in his own home or office, is not a subject of Fourth Amendment protection. But what he seeks to preserve as private, even in an area accessible to the public, may be constitutionally protected. 4 That is, the
capacity to claim the protection of the Amendment depends not upon a property right in the invaded place but upon whether the area was one in which there was reasonable expectation of freedom from governmental intrusion. 5
Katz’s focus on privacy was revitalized in Kyllo v. United States, 6 in which the Court invalidated the warrantless use of a thermal imaging device directed at a private home from a public street. The rule devised by the Court to limit police use of new technology that can
shrink the realm of guaranteed privacy is 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. 7 Relying on Katz, the Court rejected as
mechanical the Government’s attempted distinction between off-the-wall and through-the-wall surveillance. Permitting all off-the-wall observations, the Court observed,
would leave the homeowner at the mercy of advancing technology – including technology that could discern all human activity in the home. To some extent, the Court has grounded this concern about expectations of privacy in "Founding-era understandings,"  8 explaining that the Fourth Amendment "seeks to secure 'the privacies of life' against 'arbitrary power,'"  9 and that "a central aim of the Framers was 'to place obstacles in the way of a too permeating police surveillance.'"  10
Although the sanctity of the home has been strongly reaffirmed, protection of privacy in other contexts becomes more problematic. A two-part test that Justice Harlan suggested in Katz often provides the starting point for analysis. 11 The first element, the
subjective expectation of privacy, has largely dwindled as a viable standard, because, as Justice Harlan noted in a subsequent case,
our expectations, and the risks we assume, are in large part reflections of laws that translate into rules the customs and values of the past and present. 12 As for the second element, whether one has a
legitimate expectation of privacy that society finds
reasonable to recognize, the Court has said that
[l]egitimation of expectations of privacy by law must have a source outside of the Fourth Amendment, either by reference to concepts of real or personal property law or to understandings that are recognized and permitted by society. 13
Thus, protection of the home is at the apex of Fourth Amendment coverage because of the right associated with ownership to exclude others; 14 but ownership of other things, i.e., automobiles, does not carry a similar high degree of protection. 15 That a person has taken normal precautions to maintain his privacy, that is, precautions customarily taken by those seeking to exclude others, is usually a significant factor in determining legitimacy of expectation. 16 On the other hand, the Court has held that "a person has no legitimate expectation of privacy in information he voluntarily turns over to third parties."  17 Some expectations, the Court has held, are simply not among those that society is prepared to accept. 18 In the context of norms for the use of rapidly evolving communications devices, the Court was reluctant to consider
the whole concept of privacy expectations at all, preferring other decisional grounds:
The judiciary risks error by elaborating too fully on the Fourth Amendment implications of emerging technology before its role in society has become clear. 19
What seems to have emerged is a balancing standard that requires
an assessing of the nature of a particular practice and the likely extent of its impact on the individual’s sense of security balanced against the utility of the conduct as a technique of law enforcement. Whereas Justice Harlan saw a greater need to restrain police officers through the warrant requirement as the intrusions on individual privacy grow more extensive, 20 the Court’s solicitude for law enforcement objectives frequently tilts the balance in the other direction.
Application of this balancing test, because of the Court’s weighing of law enforcement investigative needs, 21 and its subjective evaluation of privacy needs, has led to the creation of a two-tier or sliding-tier scale of privacy interests. The privacy test was originally designed to permit a determination that an interest protected by the Fourth Amendment had been invaded. 22 If it had been, then ordinarily a warrant was required, subject only to the narrowly defined exceptions, and the scope of the search under those exceptions was
strictly tied to and justified by the circumstances which rendered its initiation permissible. 23 But the Court now uses the test to determine whether the interest invaded is important or persuasive enough so that a warrant is required to justify it; 24 if the individual has a lesser expectation of privacy, then the invasion may be justified, absent a warrant, by the reasonableness of the intrusion. 25 Exceptions to the warrant requirement are no longer evaluated solely by the justifications for the exception, e.g., exigent circumstances, and the scope of the search is no longer tied to and limited by the justification for the exception. 26 The result has been a considerable expansion, beyond what existed prior to Katz, of the power of police and other authorities to conduct searches.
The Berger and Katz Cases
In Berger v. New York, 27 the Court confirmed the obsolescence of the alternative holding in Olmstead that conversations could not be seized in the Fourth Amendment sense. 28 Berger held unconstitutional on its face a state eavesdropping statute under which judges were authorized to issue warrants permitting police officers to trespass on private premises to install listening devices. The warrants were to be issued upon a showing of
reasonable ground to believe that evidence of crime may be thus obtained, and particularly describing the person or persons whose communications, conversations or discussions are to be overheard or recorded. For the five-Justice majority, Justice Clark discerned several constitutional defects in the law.
First, . . . eavesdropping is authorized without requiring belief that any particular offense has been or is being committed; nor that the ‘property’ sought, the conversations, be particularly described.
The purpose of the probable-cause requirement of the Fourth Amendment to keep the state out of constitutionally protected areas until it has reason to believe that a specific crime has been or is being committed is thereby wholly aborted. Likewise the statute’s failure to describe with particularity the conversations sought gives the officer a roving commission to ‘seize’ any and all conversations. It is true that the statute requires the naming of ‘the person or persons whose communications, conversations or discussions are to be overheard or recorded. . . .’ But this does no more than identify the person whose constitutionally protected area is to be invaded rather than ‘particularly describing’ the communications, conversations, or discussions to be seized. . . . Secondly, authorization of eavesdropping for a two-month period is the equivalent of a series of intrusions, searches, and seizures pursuant to a single showing of probable cause. Prompt execution is also avoided. During such a long and continuous (24 hours a day) period the conversations of any and all persons coming into the area covered by the device will be seized indiscriminately and without regard to their connection with the crime under investigation. Moreover, the statute permits . . . extensions of the original two-month period – presumably for two months each – on a mere showing that such extension is ‘in the public interest.’. . . Third, the statute places no termination date on the eavesdrop once the conversation sought is seized. . . . Finally, the statute’s procedure, necessarily because its success depends on secrecy, has no requirement for notice as do conventional warrants, nor does it overcome this defect by requiring some showing of special facts. On the contrary, it permits unconsented entry without any showing of exigent circumstances. Such a showing of exigency, in order to avoid notice, would appear more important in eavesdropping, with its inherent dangers, than that required when conventional procedures of search and seizure are utilized. Nor does the statute provide for a return on the warrant thereby leaving full discretion in the officer as to the use of seized conversations of innocent as well as guilty parties. In short, the statute’s blanket grant of permission to eavesdrop is without adequate judicial supervision or protective procedures.  29