Katz v. United States, decided by the Supreme Court in 1967, held that the Fourth Amendment protects people, not places. The government had recorded Charles Katz’s phone booth conversations without a warrant, arguing that because agents never physically entered the booth there was no search. The Court rejected that reasoning, overruled its earlier trespass-based approach, and — through Justice Harlan’s concurrence — gave courts the two-part “reasonable expectation of privacy” test that still governs whether government conduct counts as a search more than half a century later.
What Happened to Charles Katz
Katz was charged under the federal Wire Act for transmitting gambling information by telephone across state lines from a public phone booth in Los Angeles.1Office of the Law Revision Counsel. 18 USC 1084 – Transmission of Wagering Information FBI agents attached an electronic listening device to the outside of the booth and recorded his calls. They never entered the booth itself.
The lower court upheld the conviction. Its reasoning tracked the standard of the day: no physical penetration, no Fourth Amendment problem. The Supreme Court reversed.
The Rule Katz Replaced
For nearly forty years, Fourth Amendment law had been governed by Olmstead v. United States (1928). Olmstead held that wiretapping phone lines was not a search because agents never entered anyone’s house or office. The Court described the evidence as “secured by the use of the sense of hearing and that only.”2Legal Information Institute. Olmstead v. United States, 277 US 438 Under this trespass doctrine, the amendment only kicked in when agents physically invaded a protected space.
Katz dismantled the framework. The Court held that the Fourth Amendment “protects people, not places.”3Legal Information Institute. Constitution Annotated – Katz and the Reasonable Expectation of Privacy Test By closing the door of the booth, Katz had shown he meant to keep the call private. That the booth was glass and sat on a public sidewalk did not matter. What mattered was that he had a right to assume his words would not be broadcast to the world by government agents.
Constitutional protection no longer turned on whether agents committed a physical trespass. It turned on whether the person being surveilled had a privacy interest the law should respect.
Justice Harlan’s Two-Part Test
The majority opinion set out the principle. Justice Harlan’s concurrence gave courts a workable test, and it is his formulation that judges actually apply.4Justia. Katz v. United States, 389 US 347 (1967) Government conduct amounts to a Fourth Amendment search when two conditions are met.
First, the person must have actually expected privacy. This is the subjective prong. Katz closing the booth door was a concrete act showing he believed the conversation would stay between him and the person on the other end.
Second, that expectation must be one society is prepared to recognize as reasonable. This is the objective prong. Someone shouting across a crowded restaurant might wish for privacy, but no court would treat that wish as reasonable.
Both prongs must be satisfied. The test’s usefulness is its flexibility: it doesn’t depend on the walls of a home or a property line, which is why it has survived the move from phone booths to smartphones.
How the Test Draws Lines
The home receives the strongest protection. So does its curtilage, meaning the area immediately around the home such as a porch, a fenced yard, or an attached garage. Courts weigh how close the area is to the house, whether it sits within an enclosure, what it is used for, and what the resident did to shield it from public view. A fenced backyard almost always qualifies. A detached barn hundreds of yards away across an open field is much harder to classify.
Outside the curtilage, the open fields doctrine holds that privately owned land gets no Fourth Amendment protection, even if the owner has posted signs or built fences.5Legal Information Institute. Open Field Doctrine The reasoning is that undeveloped land doesn’t carry the intimate associations of a home. Some state constitutions reject the doctrine and give private land more protection, but as a matter of federal law it still stands.
The Third-Party Doctrine Limit
One of the biggest limits on Katz is the third-party doctrine. If you voluntarily hand information to someone else, the Fourth Amendment generally stops protecting it. The Supreme Court built the rule in two cases that followed Katz.
United States v. Miller (1976) held that bank customers have no reasonable expectation of privacy in their financial records. Checks and deposit slips were “negotiable instruments to be used in commercial transactions,” and by sharing them with the bank the customer assumed the risk that the bank might turn them over to the government.6Justia. United States v. Miller, 425 US 435 (1976)
Smith v. Maryland (1979) extended that logic to telephone dialing information. People who dial phone numbers know they are routing them through the phone company’s equipment, so they assume the risk that the company could share the numbers with police. A pen register recording only dialed numbers was not a search, and no warrant was needed.7Justia. Smith v. Maryland, 442 US 735 (1979)
The doctrine made sense in an age of paper checks and rotary phones. Applied to a world where nearly every digital interaction passes through a third-party server, it has become one of the most contested pieces of Fourth Amendment law.
Katz in the Digital Age
Katz has been the launching point for the Supreme Court’s answers to modern surveillance questions.
Kyllo v. United States (2001)
Police used a thermal imaging device from a public street to detect heat patterns inside a home, patterns that pointed to high-intensity grow lamps for marijuana. In a 5-4 decision, the Court held that using technology “not in general public use” to explore details of a home that would otherwise be unknowable without physical entry is a search that requires a warrant.8Justia. Kyllo v. United States, 533 US 27 (2001) The case applied Katz’s core insight: privacy protection must keep pace with surveillance technology, or the amendment becomes a dead letter.
United States v. Jones (2012)
Police attached a GPS tracker to a suspect’s car without a valid warrant and followed his movements for 28 days. The Court unanimously called this a search, but the majority relied on the older trespass theory rather than Katz. Physically placing the device on the car was a trespass on the suspect’s property, and that alone triggered protection. The Court made clear that “the Katz reasonable-expectation-of-privacy test has been added to, but not substituted for, the common-law trespassory test.”9Legal Information Institute. United States v. Jones, 565 US 400 (2012) Katz expanded the amendment’s reach; it did not erase the older property-based route.
Riley v. California (2014)
The Court unanimously held that police generally cannot search the digital contents of a cell phone seized during an arrest without a warrant.10Justia. Riley v. California, 573 US 373 (2014) The usual “search incident to arrest” exception did not apply, because a phone is nothing like a wallet or a cigarette pack. It carries a vast archive of personal life.
Carpenter v. United States (2018)
The government obtained 127 days of historical cell-site location records without a warrant, tracking a suspect by identifying which towers his phone had connected to. In a 5-4 decision, the Court held that accessing that data was a Fourth Amendment search requiring a warrant supported by probable cause.11Supreme Court of the United States. Carpenter v. United States, 585 US 296 (2018) The Court refused to extend the third-party doctrine to cell-site records, describing “a world of difference” between the limited records in Miller and Smith and the exhaustive location chronicle a modern phone generates automatically. Carpenter did not overrule Miller or Smith, and the Court stressed that its holding was narrow. Still, it drew a clear boundary: when technology produces a pervasive, automatic, and deeply revealing record of a person’s life, the old “you shared it, so you lost it” rule no longer applies without limit.
The Warrant Requirement Katz Imposed
Katz did more than expand what counts as a search. It also insisted that the warrant requirement applies to electronic surveillance. The FBI agents in Katz had probable cause, limited their recording to specific gambling calls, and kept the surveillance short. The Court noted all of that and still found the search unconstitutional. The restraint “was imposed by the agents themselves, not by a judicial officer,” and the Fourth Amendment does not let officers be their own judges of when surveillance is justified.4Justia. Katz v. United States, 389 US 347 (1967)
Congress responded by passing Title III of the Omnibus Crime Control and Safe Streets Act of 1968, the federal wiretap statute. A wiretap order is harder to get than a routine search warrant. The application must be written, sworn, and detailed, identifying the specific crime, the communications to be intercepted, the people involved, and why normal investigative methods have failed or would be too dangerous.12Office of the Law Revision Counsel. 18 USC 2518 – Procedure for Interception of Wire, Oral, or Electronic Communications A judge can approve the order only after finding probable cause on each of those points.
For stored electronic communications like emails and cloud files, the Stored Communications Act (part of the Electronic Communications Privacy Act of 1986) sets a tiered system. Some records need only a subpoena; some need a special court order; the most sensitive content needs a full warrant.13Bureau of Justice Assistance. Electronic Communications Privacy Act of 1986 (ECPA) The idea tracks Katz: the more personal and revealing the information, the higher the legal bar to obtain it.
What Happens When the Rule Is Broken
If the government intercepts communications in violation of the wiretap statute, the target can move to suppress both the intercepted content and any evidence derived from it. The statute lists three grounds: the communication was unlawfully intercepted, the authorization order was facially insufficient, or the interception did not conform to what the order authorized.12Office of the Law Revision Counsel. 18 USC 2518 – Procedure for Interception of Wire, Oral, or Electronic Communications Suppression is often decisive; if the wiretap evidence was central to the case, the charges frequently fall with it.
Victims of illegal surveillance can also sue for money damages. Available relief includes actual damages plus any profits the violator made, or statutory damages of $10,000 or $100 per day of the violation, whichever is greater. Courts can add punitive damages in appropriate cases and order the violator to pay the victim’s attorney’s fees.14Office of the Law Revision Counsel. 18 USC 2520 – Recovery of Civil Damages Authorized The suit must be filed within two years of when the victim first had a reasonable opportunity to discover the violation.