Katz v. United States, decided by the Supreme Court in 1967, held that the FBI violated the Fourth Amendment when it recorded Charles Katz’s phone booth calls without a warrant, and it replaced the old rule that tied constitutional privacy to physical trespass with a broader principle: the Fourth Amendment “protects people, not places.”1Justia. Katz v. United States The 7-1 ruling produced the reasonable expectation of privacy test that courts still use to decide whether government surveillance counts as a search.
What Happened in the Phone Booth
Charles Katz was a Los Angeles gambler who used a public telephone booth to call bookmakers in Boston and Miami, transmitting bets across state lines in violation of federal law.2Office of the Law Revision Counsel. 18 US Code 1084 – Transmission of Wagering Information; Penalties FBI agents suspected him and attached a small electronic listening device to the outside of the booth’s glass wall. The device picked up his side of the conversations. Agents never entered the booth and never touched the phone. Based on the recordings, Katz was convicted on an eight-count federal indictment.1Justia. Katz v. United States
Katz argued the recordings were the product of an unconstitutional search. The government pointed out that the device stayed on the outside of the booth, so no physical intrusion had occurred. That distinction was the whole ballgame under existing law.
The Rule Before Katz
In 1928, the Supreme Court decided Olmstead v. United States, a Prohibition case where federal agents wiretapped a bootlegger’s phone lines from public streets and a building basement without ever entering his property. The Court ruled the Fourth Amendment did not apply because there had been no physical trespass on the defendant’s property.3Justia. Olmstead v. United States
That logic became the “trespass doctrine,” and it controlled Fourth Amendment law for nearly four decades. If the government did not physically break into your home, open your mail, or handle your belongings, it was not conducting a search. Wiretaps, hidden microphones, and other electronic surveillance sat outside the Constitution’s reach so long as agents kept their hands off your property. By the 1960s, recording technology had raced past anything the Olmstead Court could have imagined, and the government could gather enormous amounts of private information without ever crossing a property line.
What the Court Decided
The Supreme Court reversed Katz’s conviction. Justice Potter Stewart, writing for the majority, said the Fourth Amendment “protects people, not places,” and its reach “cannot turn on the presence or absence of a physical intrusion into any given enclosure.” With that, the Court dismantled the trespass rule that had governed surveillance law since Olmstead. The majority stated directly that the foundations of Olmstead could “no longer be regarded as controlling.”1Justia. Katz v. United States
The vote was 7-1. Justice Thurgood Marshall, newly appointed, did not participate. The seven justices in the majority agreed that Katz had a right to privacy inside the booth, that recording him without a warrant was an unreasonable search, and that the evidence should have been excluded. Law enforcement could no longer avoid the warrant requirement simply by choosing technology that did not physically touch a suspect’s property.
The Reasonable Expectation of Privacy Test
The most durable part of Katz did not come from Justice Stewart’s majority opinion. Justice John Marshall Harlan wrote a concurrence laying out a two-part test for deciding when a government action qualifies as a “search,” and it is Harlan’s framework, not Stewart’s broader language, that courts still apply.4Constitution Annotated. Amdt4.3.3 Katz and Reasonable Expectation of Privacy Test
Harlan required two things. First, the person must have shown an actual, subjective expectation of privacy through some concrete step, the way Katz shut the phone booth door. Second, the expectation must be one that society is prepared to recognize as reasonable. A personal wish for secrecy is not enough if most people would not share that expectation under the same circumstances. Harlan’s own formulation was “that a person have exhibited an actual (subjective) expectation of privacy and, second, that the expectation be one that society is prepared to recognize as ‘reasonable.'”1Justia. Katz v. United States
When both prongs are met, the government’s intrusion counts as a search and generally requires a warrant supported by probable cause. Evidence gathered through a warrantless search that violates this standard is generally inadmissible under the exclusionary rule.5Justia. Mapp v. Ohio
Where the Test Finds Privacy
The phone booth is the classic example. Step inside, close the door, and both prongs are satisfied. Courts have extended the same reasoning to sealed letters, private conversations in homes, and personal items kept in locked containers. The nature of the privacy interest matters more than the physical location.
Where the Test Does Not Find Privacy
Not everything a person wants to keep private actually qualifies. The Supreme Court has held that garbage left on the curb for collection carries no reasonable expectation of privacy because it sits open to anyone who walks by.6Oyez. California v. Greenwood Activities visible from public airspace or open fields beyond the area immediately surrounding a home generally fall outside the test as well. The objective prong does real work: if society would not recognize the expectation, the Fourth Amendment does not attach, no matter how strongly the individual felt.
The Third-Party Doctrine Limit
Twelve years after Katz, the Court narrowed the privacy test with a significant carve-out. In Smith v. Maryland (1979), the Court held that a person has no reasonable expectation of privacy in information voluntarily turned over to a third party. Police had used a device to log the phone numbers a suspect dialed, and the Court reasoned that because the caller voluntarily shared those numbers with the phone company to complete the calls, he assumed the risk that the company might give them to law enforcement.7Justia. Smith v. Maryland
This “third-party doctrine” opened a wide gap in Fourth Amendment protection. Bank records, phone logs, utility records, and other data shared with businesses in the course of daily life could be obtained without a warrant. The doctrine mostly sat in the background for decades. Then the digital age arrived and turned nearly every aspect of daily life into data held by some third-party company, making the tension between Katz and Smith the central privacy question of the 21st century.
Justice Black’s Dissent
Justice Hugo Black was the sole dissenter. A committed textualist, he argued the Fourth Amendment protects “persons, houses, papers, and effects,” and that conversations are none of those. You cannot search a conversation. You cannot seize spoken words. They have no “size, form, and weight.”1Justia. Katz v. United States
Black also noted that eavesdropping existed when the Framers wrote the Bill of Rights and that its omission from the amendment’s text was, in his view, deliberate. He accused the majority of rewriting the Constitution to invent a general privacy right the drafters never intended and warned that judges should not stretch constitutional language to fit changing times.
The dissent lost, and nearly six decades of Fourth Amendment doctrine have been built on the framework Black rejected. But his concern about judges expanding text beyond its original meaning has resurfaced. In United States v. Jones (2012), several justices expressed unease with the Katz test’s open-ended reliance on shifting social expectations, and the Court revived a version of the physical trespass analysis alongside Katz.8Justia. United States v. Jones The old doctrine Katz was supposed to bury still has a pulse.
How Katz Shapes Digital Privacy Today
Three modern cases show how far the Katz framework reaches.
GPS Tracking: United States v. Jones (2012)
Police attached a GPS device to a drug trafficking suspect’s car without a valid warrant and tracked his movements for 28 days. The Supreme Court unanimously held that this was a search. Justice Scalia’s majority opinion reached that result through the trespass theory, treating the physical attachment as an intrusion on a personal “effect.” Five justices wrote separately to say they would have reached the same result under the Katz test, noting that long-term GPS monitoring exposes an intimate picture of a person’s life that society recognizes as private.8Justia. United States v. Jones After Jones, the Katz test operates alongside the older trespass analysis rather than replacing it.
Cell Phone Searches: Riley v. California (2014)
Police had long been allowed to search items found on a person during an arrest without a warrant, including wallets and address books. When officers began applying that rule to smartphones, the Court drew a line. Chief Justice Roberts, writing for a unanimous Court, held that police generally need a warrant before searching the digital contents of a cell phone seized during an arrest. Modern phones, the opinion said, hold “the privacies of life” in a way no physical pocket item ever could. The Court’s instruction to officers was blunt: “Get a warrant.”9Justia. Riley v. California
Cell-Site Location Data: Carpenter v. United States (2018)
Carpenter put the Katz test in direct conflict with the third-party doctrine. The FBI obtained 127 days of historical cell-site location records from Timothy Carpenter’s wireless carrier without a warrant, using them to place him near several robberies. The government argued that because Carpenter had shared his location with his phone company by carrying a phone that connects to towers, the third-party doctrine applied.
The Court disagreed, 5-4. Chief Justice Roberts wrote that cell-site location information provides “an intimate window into a person’s life” and that “there is a world of difference between the limited types of personal information” in earlier third-party cases and the “exhaustive chronicle of location information casually collected by wireless carriers.”10Justia. Carpenter v. United States The Court also questioned whether people truly volunteer that location data at all. A cell phone logs its position automatically, hundreds of times a day, just by being on, and disconnecting from the network entirely is the only way to stop it. Carrying a phone, the Court acknowledged, is essentially a requirement of modern life.
Carpenter did not overturn the third-party doctrine outright, but it carved out a significant exception for digital records that reveal the comprehensive details of a person’s movements. The government now generally needs a warrant to obtain historical cell-site location data.10Justia. Carpenter v. United States How far that reasoning extends to other digital records held by third parties is still being worked out in the lower courts.
Why the Test Still Matters
The reasonable expectation of privacy test is nearly 60 years old, and it governs nearly every case where someone argues the government invaded their privacy without a warrant. Smart-home recordings, email searches, location tracking, facial recognition, cloud storage: the first legal question in each of these areas is whether the government’s action violated a reasonable expectation of privacy under Harlan’s two-part standard.
The test has critics. Its dependence on what “society is prepared to recognize as reasonable” creates a moving target. As people grow used to surveillance cameras, oversharing on social media, and pervasive data collection, the baseline of what counts as a reasonable expectation of privacy can shrink. Some scholars and judges have flagged a circularity problem: the government could normalize surveillance and then argue that no one reasonably expects privacy anymore. Carpenter pushed back against that spiral, but the tension has not been resolved. What began in a Los Angeles phone booth continues to define the boundary between government power and personal privacy in ways Charles Katz could never have predicted.