"[6] In 1965, Katz regularly used a public telephone booth near his apartment on Sunset Boulevard in Los Angeles to communicate his gambling handicaps to bookmakers in Boston and Miami.

[6] Unbeknownst to Katz, the FBI had begun investigating his gambling activities and was recording his conversations via a covert listening device attached to the outside of the phone booth.

But what he seeks to preserve as private, even in an area accessible to the public, may be constitutionally protected.The Supreme Court then surveyed the history of American jurisprudence on governmental searches and seizures.

In their legal briefs, the parties had focused on the 1928 precedent Olmstead v. United States, in which the Court ruled that surveillance by wiretap without any trespass did not constitute a "search" for Fourth Amendment purposes.

However, the Court stated that in later cases it had begun recognizing that the Fourth Amendment applied to recorded speech obtained without any physical trespassing, and that the law had evolved.

[14] Stewart wrote: We conclude that the underpinnings of Olmstead [and similar cases] have been so eroded by our subsequent decisions that the "trespass" doctrine there enunciated can no longer be regarded as controlling.

The Government's activities in electronically listening to and recording the petitioner's words violated the privacy on which he justifiably relied while using the telephone booth and thus constituted a "search and seizure" within the meaning of the Fourth Amendment.Stewart then concluded the Court's opinion by ruling that even though the FBI knew there was a "strong probability" that Katz was breaking the law when using the phone booth, their use of the wiretap was an unconstitutional search because they did not obtain a warrant before placing the listening device.

Thus a man's home is, for most purposes, a place where he expects privacy, but objects, activities, or statements that he exposes to the "plain view" of outsiders are not "protected" because no intention to keep them to himself has been exhibited.

[3] The Supreme Court's decision in Katz significantly expanded the scope of the Fourth Amendment's protections, and represented an unprecedented shift in American search and seizure jurisprudence.

The Katz test of an objective "reasonable expectation of privacy," which has been widely adopted by U.S. courts, has proven much more difficult to apply than the traditional analysis of whether a physical intrusion into "persons, houses, papers, and effects" occurred.

[...] Although four decades have passed since Justice Harlan introduced the test in his concurrence in Katz v. United States, the meaning of the phrase "reasonable expectation of privacy" remains remarkably opaque.