Master Landmark Fourth Amendment case establishing the reasonable expectation of privacy standard and rejecting a purely trespass-based approach. with this comprehensive case brief.
Katz v. United States transformed Fourth Amendment doctrine by shifting the focus from places and physical trespass to people and their privacy. Before Katz, the Court often tethered Fourth Amendment protections to property concepts, suggesting that without a physical intrusion into a constitutionally protected area, there was no search. Katz rejected that framework, holding that what the Fourth Amendment protects are the privacy expectations individuals reasonably maintain against government intrusion, not simply the sanctity of physical spaces.
This case is foundational for understanding modern search-and-seizure law, especially in the context of electronic surveillance and digital privacy. It introduced the reasonable expectation of privacy standard, later crystallized in Justice Harlan’s concurrence into a widely cited two-part test. Katz continues to guide courts in applying the Fourth Amendment to technologies unknown to the Framers, shaping outcomes from wiretaps to thermal imaging and cell-site location data.
389 U.S. 347 (1967) (U.S. Supreme Court)
The FBI suspected Charles Katz of transmitting wagering information by telephone in violation of federal gambling laws. Katz regularly used a series of public telephone booths in Los Angeles to place calls to contacts in Boston and Miami. Without obtaining a warrant, FBI agents attached an electronic listening and recording device to the outside of the phone booth Katz used. The device captured Katz’s end of the conversations but did not physically penetrate the booth. At trial, the government introduced the recordings to prove Katz’s involvement in illegal gambling, and he was convicted. The Ninth Circuit affirmed, reasoning that there was no Fourth Amendment violation because there was no physical intrusion into the booth. The U.S. Supreme Court granted certiorari.
Does the warrantless electronic recording of a person’s conversation from outside a public telephone booth, where the speaker has sought to preserve privacy, constitute a search and seizure in violation of the Fourth Amendment?
The Fourth Amendment protects people, not places. A search occurs when the government violates a person’s subjective expectation of privacy that society recognizes as reasonable. Physical trespass is not required. Absent a specifically applicable exception, electronic surveillance of private conversations requires a warrant supported by probable cause and particularity.
Yes. The government’s warrantless electronic eavesdropping on Katz’s conversation in a closed public phone booth violated the Fourth Amendment. The conviction was reversed.
The Court, per Justice Stewart, emphasized that the Fourth Amendment protects the privacy of people rather than the sanctity of physical locations. Katz’s conduct—entering a phone booth, closing the door, and paying to make a call—manifested a subjective expectation of privacy in the content of his conversation. Society is prepared to recognize that expectation as reasonable, even though the booth is in a public place. The Court rejected the notion that a search requires physical penetration of a protected area, disavowing the property-centric approach associated with cases like Olmstead v. United States and Goldman v. United States. The government’s attachment of a listening device to the exterior of the booth and its recording of Katz’s words intruded upon the privacy of his communication and thus constituted a “search and seizure” within the meaning of the Fourth Amendment. The Court also explained that when the government seeks to intrude upon such privacy, it must obtain a warrant, unless a recognized exception applies. No exigent circumstances or other exceptions justified the FBI’s surveillance here, and the agents could have pursued a warrant tailored to the particular conversations and times at issue. Justice Harlan’s concurrence articulated the now-canonical two-prong test: (1) the individual exhibited an actual (subjective) expectation of privacy, and (2) that expectation is one society is prepared to recognize as reasonable. Applying that framework, Katz’s expectation was both demonstrated and reasonable. Justice White concurred to underscore that national security wiretaps might present different questions. Justice Black dissented, arguing that the text of the Fourth Amendment does not cover eavesdropping on words absent a physical search of “persons, houses, papers, and effects.”
Katz is the cornerstone of modern Fourth Amendment law. It recast the definition of a search in terms of reasonable expectations of privacy, enabling the Constitution to regulate emerging forms of surveillance without requiring a physical trespass. The decision directly informs analysis of electronic eavesdropping and has been extended to technologies like thermal imaging (Kyllo v. United States) and cell-site location data (Carpenter v. United States). At the same time, Katz coexists with doctrines limiting privacy expectations, such as the third-party doctrine (Smith v. Maryland; United States v. Miller), and with renewed attention to trespass as an additional, not exclusive, basis for finding a search (United States v. Jones). For law students, Katz provides the analytic template for Fourth Amendment questions: identify the individual’s asserted privacy interest, assess whether the person exhibited a subjective expectation, evaluate whether that expectation is objectively reasonable, and determine whether a warrant or exception applies. It also influenced legislative responses, notably Title III of the Omnibus Crime Control and Safe Streets Act of 1968, which set procedures for wiretaps consistent with Katz’s requirements.
Justice Harlan’s concurrence distilled the test into two prongs: (1) the individual exhibited an actual, subjective expectation of privacy, and (2) that expectation is one society is prepared to recognize as reasonable. If both are satisfied and the government intrudes without a warrant or valid exception, there is a Fourth Amendment violation.
No. Katz expressly rejects a physical trespass requirement. Electronic eavesdropping and other non-trespassory surveillance can be searches if they invade a reasonable expectation of privacy. Physical trespass remains relevant after United States v. Jones, but it is an alternative basis for finding a search, not a prerequisite.
Olmstead tied Fourth Amendment protection to property and physical intrusion. Katz overruled that approach by holding that the Amendment protects people’s privacy interests even in public places and without physical penetration. This pivot allowed courts to regulate electronic surveillance based on privacy expectations rather than trespass alone.
Katz’s framework underlies decisions like Kyllo (thermal imaging of a home is a search) and Carpenter (historical cell-site location information generally requires a warrant). Courts assess whether there is a reasonable expectation of privacy in the data, mindful of how pervasive and revealing modern technologies can be.
Not always. While Katz generally requires a warrant for surveillance that intrudes on a reasonable expectation of privacy, recognized exceptions (e.g., exigent circumstances, consent) may apply. Moreover, under the third-party doctrine, information voluntarily conveyed to third parties (e.g., dialed numbers in Smith v. Maryland) may fall outside Katz’s protection, though Carpenter narrowed this in the context of certain sensitive digital records.
Katz v. United States reoriented Fourth Amendment analysis around privacy rather than property. By recognizing that a person who enters a phone booth, closes the door, and pays to make a call seeks to preserve the privacy of their conversation, the Court announced that the Constitution protects reasonable expectations of privacy, even in spaces accessible to the public.
This principle remains vital in an era of rapid technological change. Katz equips courts and advocates with a flexible, privacy-centered standard for evaluating government surveillance, ensuring that constitutional protections keep pace with innovations from wiretaps to smartphones and beyond.