A listening device taped to the outside of a phone booth started it. In December 1967 the Supreme Court held in Katz v. United States (389 U.S. 347) that the warrantless use of that device broke the Fourth Amendment, and in doing so it dropped the old requirement of a physical trespass. The amendment, the Court said, protects people, not places.
The test that governs searches today came from a separate opinion. Justice John Harlan, concurring, set out two questions. Did the person actually, subjectively expect privacy? And is that an expectation society is prepared to treat as reasonable? If both answers are yes and the government intrudes, a search has happened and a warrant is normally needed.
Where the line falls
The majority in Katz put the principle plainly: what a person knowingly exposes to the public, even in his own home or office, is not protected, but what he seeks to keep private, even in a public place, may be. Later cases sorted out what that means in practice.
The home sits at the top. Protection there rests on the right of an owner to keep others out, and in Kyllo v. United States (2001) the Court ruled that pointing a thermal imaging device at a house from the street was presumptively unreasonable without a warrant. The justices worried that otherwise advancing police technology would leave homeowners at its mercy. Cars get less protection. Closed luggage gets more than a car does, though less once it is inside a vehicle police have probable cause to search.
Plenty of things fall outside. The Court found no legitimate expectation of privacy in bank records (United States v. Miller, 1976), numbers dialed from a phone (Smith v. Maryland, 1979), a prison cell (Hudson v. Palmer, 1984) or garbage in sealed bags left at the curb (California v. Greenwood, 1988). Open fields are not protected under federal law either, though some states grant protection, according to Cornell’s Legal Information Institute. The pattern behind Miller and Smith is the third-party doctrine: information handed voluntarily to someone else loses protection.
The Constitution Annotated, the Library of Congress guide to the document, adds a caution. The first prong, the person’s own expectation, has largely dwindled as a working standard, because expectations track what the law already permits. The second prong carries the weight. Citing Rakas v. Illinois (1978), the Court has said society’s recognition must have a source outside the Fourth Amendment, such as property law or accepted understandings.
The phone problem
Phones strained the third-party doctrine. In 2012 Justice Sonia Sotomayor wrote in United States v. Jones that it was ill-suited to a time when people reveal so much about themselves in routine tasks. In Carpenter v. United States (2018) the Court agreed in part. It declined to extend Smith and Miller to cell-site records, holding that a person keeps a legitimate expectation of privacy in the record of his physical movements.
That line was tested again this year. On June 29, 2026, the Court ruled 6-3 in Chatrie v. United States that police obtaining an individual’s cell location data, even from a tech company, conduct a Fourth Amendment search, according to The Conversation. The case came from a May 2019 bank robbery in Midlothian, Virginia, where a geofence warrant made Google identify every phone in a 17½-acre area for two hours. The justices sent the case back to the 4th Circuit to decide whether the warrant met the Constitution’s requirements.
Sources
- expectation of privacy | Wex, Legal Information Institute (Cornell)
- Supreme Court rules your cellphone location data is protected by the Fourth Amendment | The Conversation
