In Katz v. United States, decided December 18, 1967, the Supreme Court ruled 7-1 that the Fourth Amendment “protects people, not places,” and that the government conducts a search whenever it violates a privacy interest a person reasonably holds, regardless of whether agents physically trespass. The reasonable expectation of privacy test that emerged from Justice Harlan’s concurrence in Katz v. United States asks two questions: did the person actually expect privacy, and would society recognize that expectation as reasonable. If both are yes, the government generally needs a warrant.1Justia U.S. Supreme Court Center. Katz v. United States
What Happened to Charles Katz
Charles Katz was a professional gambler in Los Angeles who used a particular public telephone booth to place bets with bookmakers in other states. FBI agents suspected him of violating the federal wire wagering statute. To gather evidence, they attached a small microphone and recording device to the outside of the booth. They never entered it and never tampered with its interior. The recordings of Katz’s side of the calls were used to convict him on eight counts, and the lower courts upheld the conviction on the theory that because agents stayed outside the glass, no “search” had occurred.1Justia U.S. Supreme Court Center. Katz v. United States
The Rule the Court Replaced
For nearly four decades before Katz, courts followed Olmstead v. United States (1928), which held that wiretapping did not count as a search because agents never physically entered a home or office. Olmstead read the Fourth Amendment’s reference to “persons, houses, papers, and effects” as limited to tangible things and physical places. A phone conversation was intangible, and connections made on public streets or in shared basements involved no trespass, so no constitutional violation occurred.2Library of Congress. Olmstead v. United States, 277 U.S. 438 (1928)
That approach gave law enforcement wide latitude as surveillance technology improved. Justice Potter Stewart, writing for the Katz majority, declared the Olmstead trespass rule “no longer controlling.” By closing the door of the booth, Katz had shown he intended to keep his conversation private. The booth’s glass walls, its public location, and the fact that the phone company owned it were all irrelevant. The FBI could have sought a warrant on the evidence it already had. It didn’t, and the surveillance was therefore unconstitutional.1Justia U.S. Supreme Court Center. Katz v. United States
The Two-Part Test
The majority stated the principle, but Justice John Marshall Harlan’s concurrence supplied the specific test courts still use today. Both prongs must be satisfied before Fourth Amendment protection attaches.3Congress.gov. Amdt4.3.3 Katz and Reasonable Expectation of Privacy Test
The first prong is subjective. The person must have actually believed the activity was private, and that belief usually shows up in behavior. Katz demonstrated it by stepping into a phone booth and pulling the door shut. Someone shouting on a street corner has not.
The second prong is objective. Society must be prepared to recognize the expectation as reasonable. A judge asks whether the average person would consider the expectation justified under the circumstances. The word “reasonable” carries the weight here, and it is what most Fourth Amendment litigation fights over.
When both prongs are met, the government generally needs a warrant from a neutral judge before intruding. Evidence gathered without one can be excluded from trial under the exclusionary rule.4Constitution Annotated. Exclusionary Rule and Evidence
Where the Test Finds Privacy and Where It Doesn’t
Courts have spent decades mapping where the test locates protection and where it doesn’t. The results are not always intuitive.
The Home and Its Curtilage
The home receives the strongest protection. In Kyllo v. United States (2001), the Court held that when the government uses technology not in general public use to detect details inside a home that would otherwise require physical entry, that surveillance is a search. The case involved thermal imaging aimed at a house to detect heat lamps used for growing marijuana. The Court rejected the argument that only “intimate details” deserve protection, holding that inside the home, all details are intimate.5Justia U.S. Supreme Court Center. Kyllo v. United States
Protection extends to the curtilage, meaning the area immediately surrounding the home like a porch, yard, or attached garage. Land beyond the curtilage falls under the “open fields” doctrine and receives no Fourth Amendment protection, even if the land is privately owned and fenced.
Trash at the Curb
In California v. Greenwood (1988), the Court ruled that garbage bags left at the curb for collection carry no reasonable expectation of privacy. Trash on a public street is accessible to animals, scavengers, and passersby, and by putting it out for the collector to take away, the owner has effectively abandoned any privacy interest in what’s inside.6Justia U.S. Supreme Court Center. California v. Greenwood
Plain View
Anything visible from a lawful vantage point falls outside the test. If an officer standing on a public sidewalk sees contraband through an open window, no search has occurred. The same applies to items visible on a car seat during a lawful stop.
The Third-Party Doctrine
One consequential extension of Katz cuts the other way. Under the third-party doctrine, when you voluntarily hand information to someone else, you assume the risk that they will share it with the government. In the 1970s the Supreme Court used this reasoning to deny Fourth Amendment protection to two categories of records millions of Americans generate every day.
In United States v. Miller (1976), the Court held that bank customers have no reasonable expectation of privacy in their financial records because those records belong to the bank. In Smith v. Maryland (1979), it applied the same logic to phone numbers dialed from a home telephone: callers know the phone company records the numbers, so no legitimate expectation of privacy attaches.7Library of Congress. Smith v. Maryland, 442 U.S. 735 (1979)
For decades, that framework meant the government could obtain bank records, phone logs, and similar business records without a warrant. Then smartphones arrived.
Katz Applied to Phones and Tracking
Modern devices generate constant, detailed records held by third parties. Several recent decisions have refined how Katz applies to that reality.
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. The “search incident to arrest” exception, which lets officers check an arrestee’s pockets for weapons or destructible evidence, does not extend to digital data. Data can’t be used as a weapon, and concerns about remote wiping are better handled through narrower exceptions like exigent circumstances.8Justia U.S. Supreme Court Center. Riley v. California
United States v. Jones (2012)
Agents attached a GPS tracker to a suspect’s car and monitored his movements for 28 days. The Court held this was a Fourth Amendment search. Justice Scalia’s majority opinion relied on the physical trespass of attaching the device, and the Court clarified that Katz’s privacy test supplements the older trespass analysis rather than replacing it. Either theory can independently trigger Fourth Amendment protection.9Legal Information Institute. United States v. Jones
Carpenter v. United States (2018)
The government obtained 127 days of a suspect’s cell-site location records from his wireless carrier without a warrant, relying on a court order requiring only “reasonable grounds” rather than probable cause. In a 5-4 decision, the Court held that accessing this data was a search requiring a warrant. Chief Justice Roberts wrote that cell-site location information is fundamentally different from the bank records in Miller or the phone numbers in Smith. Carrying a phone is essentially mandatory in modern life, and the device logs its location automatically without any deliberate act by the user. The Court declined to treat this as information “voluntarily shared” with a third party, describing “a world of difference” between the limited records at issue in the 1970s cases and the exhaustive chronicle of physical movements generated by a modern smartphone.10Justia U.S. Supreme Court Center. Carpenter v. United States
How Congress Responded
The year after Katz, Congress passed Title III of the Omnibus Crime Control and Safe Streets Act of 1968, commonly called the Wiretap Act, codified at 18 U.S.C. sections 2510 through 2522. It created a detailed procedure law enforcement must follow before intercepting phone calls or other communications.11Office of the Law Revision Counsel. 18 U.S.C. Chapter 119 – Wire and Electronic Communications Interception
The requirements are deliberately burdensome. An application must be authorized by a senior Department of Justice official, filed under oath with a federal judge, and supported by a detailed statement of facts establishing probable cause. The applicant must also explain why less intrusive techniques have failed or would be unlikely to succeed. These safeguards reflect the Katz Court’s insistence that electronic surveillance demands advance judicial approval, not after-the-fact justification.