Katz v. United States — Reasonable Expectation of Privacy

Katz v. United States, 389 U.S. 347 (1967), is the Supreme Court decision that fundamentally reconceived the Fourth Amendment's scope — moving from a property-based framework to a privacy-based framework, and establishing the "reasonable expectation of privacy" test that continues to govern Fourth Amendment analysis today. Charles Katz used a public telephone booth in Los Angeles to transmit gambling information across state lines. FBI agents, suspecting illegal activity, attached an electronic listening device to the outside of the booth and recorded his end of the conversations. No warrant was obtained. The government argued that because the listening device was on the exterior of the booth (no physical trespass into the booth), the Fourth Amendment did not apply — citing Olmstead v. United States (1928), which had held that wiretapping without physical trespass was not a "search" under the amendment. The Supreme Court rejected this property-based framework. Justice Stewart's majority held that "the Fourth Amendment protects people, not places" — what a person knowingly exposes to the public is not protected, but what they seek to preserve as private, even in a public area, is protected. Justice Harlan's concurrence articulated the test that became the governing doctrine: whether the person had a subjective expectation of privacy, and whether that expectation is one that society recognizes as reasonable. Katz remains the foundation of all modern Fourth Amendment analysis, though its application to digital technology and third-party data — confronted in Carpenter v. United States (2018) — is actively evolving.

Current Law (2026)

Legal Authority

• U.S. Const. amend. IV — "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"
• 18 U.S.C. § 2511 — Federal Wiretap Act (Title III of the Omnibus Crime Control and Safe Streets Act of 1968); prohibits intentional interception of wire, oral, and electronic communications; enacted immediately after Katz to create the statutory framework the Court said was required
• 18 U.S.C. § 2518 — Wiretap warrant procedures; requires judicial authorization before law enforcement intercepts communications; implements the Katz constitutional requirement in statute
• 50 U.S.C. § 1801 et seq. — Foreign Intelligence Surveillance Act (FISA); creates a special court (FISC) for foreign intelligence electronic surveillance; a parallel regime to Title III for national security contexts
• Olmstead v. United States, 277 U.S. 438 (1928) — Wiretapping of telephone lines without physical trespass was not a Fourth Amendment "search"; the amendment protects "material things" — property — not conversations; Justice Brandeis's prophetic dissent argued the Fourth Amendment protects private communications regardless of physical intrusion
• Berger v. New York, 388 U.S. 41 (1967) — New York's eavesdropping statute authorizing electronic surveillance without individualized suspicion violated the Fourth Amendment; decided the year before Katz and foreshadowing its approach
• Katz v. United States, 389 U.S. 347 (1967) — "The Fourth Amendment protects people, not places"; physical intrusion not required; listening device on exterior of phone booth was a "search" requiring a warrant; Olmstead overruled as applied to electronic surveillance
• Smith v. Maryland, 442 U.S. 735 (1979) — No reasonable expectation of privacy in phone numbers dialed — they are voluntarily conveyed to the phone company (a third party); the "third-party doctrine" limits Katz's reach to information not shared with others
• United States v. Miller, 425 U.S. 435 (1976) — Bank records voluntarily conveyed to a bank are not protected by the Fourth Amendment; foundational third-party doctrine case
• Kyllo v. United States, 533 U.S. 27 (2001) — Use of thermal imaging technology to detect heat signatures from inside a home (indicating a marijuana grow operation) was a "search" requiring a warrant; technology that reveals details of the home interior that would otherwise be private requires a warrant
• United States v. Jones, 565 U.S. 400 (2012) — GPS tracker physically attached to a car is a Fourth Amendment search; five justices agreed that prolonged GPS surveillance also violates a reasonable expectation of privacy; significant concurrences by Alito and Sotomayor on the privacy dimensions
• Carpenter v. United States, 585 U.S. 296 (2018) — The government's warrantless acquisition of seven days of cell-site location information (CSLI) from a phone carrier violated the Fourth Amendment; the third-party doctrine does not extend to comprehensive digital records that reveal the "privacies of life" (see Carpenter v. United States)

Key Mechanics

Katz v. United States (1967) transformed the Fourth Amendment from a property-based doctrine into a reasonable expectation of privacy test. Charles Katz used a public phone booth in Los Angeles to place illegal sports bets; the FBI attached a listening device to the outside of the booth without physically entering it. Under Olmstead (1928), there was no Fourth Amendment violation because there was no physical trespass. But Justice Stewart's majority opinion held that the Fourth Amendment "protects people, not places" — what a person "seeks to preserve as private, even in an area accessible to the public, may be constitutionally protected." Justice Harlan's concurrence, which became the operative test, stated it as two requirements: (1) the individual must have exhibited a subjective expectation of privacy, and (2) society must recognize that expectation as reasonable. The FBI's surveillance — recording Katz's voice through a listening device on the booth exterior — was a "search" that required a warrant. Katz overruled Olmstead and required Congress to create a statutory warrant framework, which became the Federal Wiretap Act (Title III). The Katz test has been refined by the third-party doctrine (Smith v. Maryland, Miller): if you voluntarily share information with a third party (phone company, bank), you lose your reasonable expectation of privacy in that information — the government can obtain it without a warrant. Carpenter v. United States (2018) began narrowing the third-party doctrine for comprehensive digital location records, with the Court holding that pervasive CSLI tracking is a Fourth Amendment search despite the voluntary-disclosure rationale.

How It Works

The Pre-Katz Framework: Trespass and the Wire

Before Katz, Fourth Amendment doctrine was rooted in the common law of trespass. Olmstead v. United States (1928) crystallized the property-based approach: wiretapping telephone wires in the street outside Olmstead's home involved no physical trespass into the home, therefore no "search" within the Fourth Amendment's meaning. Chief Justice Taft's majority relied on the text of the amendment — "persons, houses, papers, and effects" — all physical things. Conversations, he held, were not material things; they were not protected.

Justice Brandeis's dissent in Olmstead became one of the most celebrated dissents in constitutional law. Brandeis argued that the Framers had conferred a general right "to be let alone" — the right "most valued by civilized men." As technology advanced, the Fourth Amendment had to evolve to preserve the freedom it was meant to protect. "The greatest dangers to liberty lurk in insidious encroachment by men of zeal, well-meaning but without understanding." Brandeis was writing about wiretapping in 1928; the passage proved equally applicable to every subsequent technology.

For four decades after Olmstead, the trespass framework governed. Police could conduct electronic surveillance without a warrant as long as no physical object crossed a property line. This meant that bugging a room by a device pressed against the outside wall was not a search; a device that penetrated the wall by a fraction of an inch was a search (Silverman v. United States, 1961). The line of the property — the trespass line — determined constitutional protection.

Stewart's Opinion: People, Not Places

Justice Stewart's majority opinion in Katz rejected the trespass framework decisively. The Fourth Amendment "protects people, not places." A person who enters a phone booth and shuts the glass door "assumes the risk of being overheard" by passersby — but not by a government listening device secretly attached to the booth's exterior. The physical enclosed space of the phone booth gave Katz a reasonable expectation that his words would not be broadcast to the world. The absence of a trespass by the listening device was irrelevant.

What mattered was whether there had been an intrusion into a protected private sphere, not whether there had been a physical trespass into a protected physical space. The government's actions — attaching a listening device and recording Katz's words — violated "the privacy upon which he justifiably relied while using the telephone booth."

Stewart did not spell out a test for identifying the "private sphere." That came from Justice Harlan.

Justice Harlan's Concurrence: The Two-Part Test

Justice Harlan's concurrence in Katz articulated the test that has become the governing standard for all subsequent Fourth Amendment cases:

First prong: The person must have exhibited a subjective expectation of privacy — they must have actually sought to keep the thing, conversation, or activity private.

Second prong: That expectation must be one that society is prepared to recognize as reasonable — it must be objectively reasonable in the circumstances.

Both prongs are necessary. A person who whispers to themselves in a crowded room expecting no one to hear has a subjective expectation of privacy, but society does not recognize that as a reasonable expectation. A person who leaves their garbage at the curb for collection has no reasonable expectation of privacy in the contents, even if they subjectively hoped no one would look (California v. Greenwood, 1988).

Harlan's two-part test — though originally a concurrence — became the majority doctrine. It is the analytical framework applied in every subsequent Fourth Amendment "search" case.

The Third-Party Doctrine: Katz's Major Limit

Smith v. Maryland (1979) established the third-party doctrine, the most significant limit on Katz's privacy framework. A person who voluntarily conveys information to a third party — a phone company, a bank, an internet service provider — assumes the risk that the third party will reveal that information to the government. There is no reasonable expectation of privacy in information voluntarily shared with others.

The third-party doctrine has two foundational cases: Miller (1976) established that bank records — checks, deposit slips — are not protected because the bank is a third party with whom the customer voluntarily shared the information. Smith (1979) established that telephone numbers dialed are not protected because the customer conveys them to the phone company to complete the call.

For decades, the third-party doctrine was a major limitation on Katz in the digital age. Email headers, internet browsing history, location data, financial records — all of these involve third-party service providers. Under a strict reading of Smith and Miller, the government could access all of this data without a warrant.

Carpenter and the Digital Reckoning

Carpenter v. United States (2018) dramatically complicated the third-party doctrine. The FBI obtained seven days of cell-site location information (CSLI) — records showing which cell towers Carpenter's phone pinged — from his carrier without a warrant, under a lower legal standard (the Stored Communications Act's "reasonable grounds" requirement rather than probable cause). The location records placed him near the sites of a series of armed robberies.

Chief Justice Roberts's majority held that the third-party doctrine did not extend to CSLI. The nature of CSLI — comprehensive, retrospective, automatically generated — was different in kind from the information in Smith and Miller. A person does not "voluntarily" share location information with a carrier in any meaningful sense; the phone automatically pings towers as a function of how cellular networks work. And the resulting record — seven days of continuous location surveillance — reveals "the privacies of life" in a way that pen register records and bank statements do not.

Carpenter applied a Katzian framework: does the acquisition of this information constitute a "search" — does it violate a reasonable expectation of privacy? Yes, for comprehensive location records. The Court explicitly declined to extend the third-party doctrine to cover the digital records of the modern era that reveal the intimate details of a person's life.

Carpenter was deliberately narrow — it addressed only CSLI, expressly declined to decide other digital third-party data questions, and left open the application of its reasoning to email content, real-time location tracking, internet browsing history, smart device data, and other categories. Post-Carpenter litigation is working through these questions across the circuits.

Katz and Technology: The Ongoing Evolution

Katz has proven adaptable across technological generations:

Thermal imaging (Kyllo v. United States, 2001): Using thermal imaging to detect heat signatures from inside a home — revealing a marijuana grow operation — was a Fourth Amendment search. Technology that enables police to gather information about the interior of a home that would otherwise require physical entry requires a warrant.

GPS tracking (United States v. Jones, 2012): Installing a GPS tracker on a car is a Fourth Amendment search (five justices agreed on this). More significantly, five concurring justices — Alito and Sotomayor writing separately — argued that extended GPS surveillance, tracking a person's movements over 28 days, violates reasonable expectations of privacy even on public streets. The duration and comprehensiveness of the surveillance is the key, not just the physical attachment of the device. This rationale, extended in Carpenter, is reshaping digital surveillance law.

Cell phone searches (Riley v. California, 2014): Police may not search the contents of a cell phone incident to arrest without a warrant. A cell phone is "a minicomputer" — it contains the "privacies of life" — and is categorically different from the physical objects officers were previously permitted to search incident to arrest.

How It Affects You

If you are a private individual: Katz means that what you communicate privately — phone calls, text messages, emails, private conversations — is protected from warrantless government interception. But the third-party doctrine creates significant limits: the records your phone company, bank, internet provider, and other services keep about your activity are generally not protected by the Fourth Amendment unless Carpenter's "privacies of life" principle applies. Practical implications: (1) Assume that records held by your service providers are accessible to the government with a subpoena or court order, not necessarily a warrant; (2) Encrypted communications that service providers cannot access provide stronger protection than unencrypted ones; (3) Location data from your phone may be accessible to law enforcement under standards lower than probable cause in many jurisdictions not covered by Carpenter.

If you are a business: Your communications with customers and your internal communications have different Fourth Amendment profiles. Information you share with third parties — banks, cloud storage providers, email hosts — may be subject to government subpoenas without triggering Fourth Amendment protection. However, the contents of your email stored with a provider and your CSLI (after Carpenter) require a warrant. Consider: (1) whether your communications are encrypted end-to-end (more protection) or accessible by your service provider (third-party doctrine risk); (2) data retention policies — the longer you retain data, the larger the database the government can potentially access; (3) the Electronic Communications Privacy Act (ECPA) provides statutory protections beyond the constitutional floor.

If you are law enforcement: Katz means that any government action that violates a reasonable expectation of privacy is a "search" requiring Fourth Amendment justification — typically a warrant supported by probable cause. Know the third-party doctrine's boundaries: phone numbers dialed (Smith), bank records (Miller), publicly observable activity are not protected. But CSLI requires a warrant (Carpenter); phone content requires a warrant (Riley); thermal imaging of homes requires a warrant (Kyllo). For novel surveillance technologies not yet addressed by the Supreme Court, the Katz test — does this violate a reasonable expectation of privacy? — is the analytical starting point. Get a warrant when in doubt.

If you are a civil liberties litigator: Katz's two-part test is the entry point for challenging any novel government surveillance program. The key arguments: (1) Did the target have a subjective expectation of privacy? (Generally easy to establish for communications and personal data.); (2) Is that expectation objectively reasonable? (This is where the fight is — does Carpenter's "privacies of life" principle extend to the data at issue?); (3) Does the third-party doctrine apply, and if so, does Carpenter's limitation on the third-party doctrine override it? The circuit courts are developing these answers for email content, social media data, smart device data, and other digital third-party records. Bring state constitutional claims in parallel — state privacy protections often exceed the federal Katz floor.

State Variations

Katz establishes the federal constitutional floor for Fourth Amendment protection. State variations are significant:

State constitutional search and seizure provisions: Many states provide broader privacy protection than Katz. California's Constitution includes an explicit right to privacy (Article I, § 1) that courts have applied more broadly than federal Fourth Amendment doctrine. Massachusetts's Declaration of Rights has been interpreted to provide broader warrant requirements for third-party records. New York's Constitution provides broader wiretapping protection.

State wiretapping laws: Most states have enacted wiretapping statutes that parallel the federal Wiretap Act, but many states impose additional requirements — including "all-party consent" requirements for intercepting communications. Federal law requires only one-party consent; state law in many states requires that all parties to a communication consent before it is recorded. This creates criminal liability for secret recording in two-party-consent states even if federal law is not violated.

State electronic surveillance statutes: Several states have enacted statutes specifically requiring warrants for data that the third-party doctrine might not protect under federal law — including email content, location data, and other digital records. California's Electronic Communications Privacy Act (CalECPA, 2016) requires a warrant for electronic data including device data, location information, and stored communications.

Pending Legislation

• Email Privacy Act: Proposals to require a warrant for email content stored with a provider, regardless of how long it has been stored; existing federal law (ECPA's 180-day rule) requires only a subpoena for email stored more than 180 days; has had broad congressional support but has not been enacted.
• Fourth Amendment is Not For Sale Act: Would close the "data broker loophole" — requiring law enforcement to obtain a warrant before purchasing data from data brokers that they could not obtain directly from tech companies without a warrant; passed the House in 2024; Senate status pending.
• FISA reform: The Foreign Intelligence Surveillance Act Section 702 was reauthorized April 20, 2024 by the Reforming Intelligence and Securing America Act (RISAA) for two years (sunset April 19, 2026); a 3-year extension passed the House in May 2026; debate over warrant requirements for querying databases containing Americans' communications obtained under FISA continues.

Recent Developments

• 2018 — Carpenter v. United States: The Supreme Court held that warrantless acquisition of seven days of CSLI violates the Fourth Amendment; Chief Justice Roberts's majority opinion limited the third-party doctrine and applied Katz's reasonable expectation framework to comprehensive digital location records.
• 2020–2024 — Post-Carpenter digital surveillance cases: Federal courts are applying Carpenter's "privacies of life" framework to email metadata, IP address records, tower dump warrants (all phones connecting to a tower), geofence warrants (all phones in a geographic area at a given time), and other digital surveillance tools; circuits are divided on whether these require warrants.
• 2023 — Warshak doctrine: The Sixth Circuit's Warshak ruling (2010) that stored email content requires a warrant has been increasingly influential post-Carpenter; courts are extending warrant requirements to other categories of stored electronic communications.
• 2024–2026 — Geofence warrants: Multiple courts have considered whether Google geofence warrants — which require Google to disclose all devices present in a geographic area during a time window — violate the Fourth Amendment; some courts have found them unconstitutional as general warrants; the Supreme Court has not yet addressed the issue.