Katz v. United States is the 1967 Supreme Court decision that redefined the Fourth Amendment by shifting its focus from physical property to personal privacy. In a 7–1 ruling, the Court held that FBI agents violated Charles Katz’s constitutional rights when they recorded his phone calls from a public telephone booth without a warrant, and it announced a new principle that the Fourth Amendment “protects people, rather than places.” The decision replaced decades of precedent that had tied constitutional protection to physical trespass, and it produced the reasonable expectation of privacy test that still governs government surveillance today.1Justia. Katz v. United States, 389 U.S. 347 (1967)
What Happened to Charles Katz
The FBI suspected Katz of running an illegal interstate gambling operation. Agents believed he was using a public phone booth to transmit betting information across state lines, a federal crime.2Office of the Law Revision Counsel. 18 U.S. Code 1084 – Transmission of Wagering Information; Penalties
Rather than apply for a warrant, agents attached an electronic listening and recording device to the outside of the booth Katz used. They captured his end of several conversations, prosecutors introduced the recordings at trial, and Katz was convicted. On appeal, the Ninth Circuit upheld the conviction on the ground that the agents had never physically entered the booth. Under the law as it stood, no physical intrusion meant no search.1Justia. Katz v. United States, 389 U.S. 347 (1967)
The Rule the Court Had to Confront
For nearly forty years, Fourth Amendment law had followed Olmstead v. United States (1928), which held that wiretapping did not count as a search because agents never entered the defendants’ homes or offices. Olmstead read the amendment as protecting tangible things — persons, houses, papers, and effects — and treated listening to a conversation through a wiretap as involving “no searching” and “no seizure.”3Justia U.S. Supreme Court Center. Olmstead v. United States, 277 U.S. 438 (1928)
By the 1960s that rule had grown increasingly out of step with the technology available to law enforcement. Electronic eavesdropping could capture the most intimate details of a person’s life without touching anything they owned. The question in Katz was whether the Constitution had anything to say about surveillance that respected property lines but obliterated personal privacy.
The Supreme Court’s Ruling
Justice Potter Stewart wrote the majority opinion. Justice Thurgood Marshall did not participate.1Justia. Katz v. United States, 389 U.S. 347 (1967)
The core of the ruling was a single principle: the Fourth Amendment “protects people, rather than places,” and “its reach cannot turn on the presence or absence of a physical intrusion into any given enclosure.” The Court declared that the trespass doctrine from Olmstead was “no longer controlling.”1Justia. Katz v. United States, 389 U.S. 347 (1967)
The Court also held that the amendment covers the recording of spoken words, not just the seizure of physical objects. That was a direct rejection of the Olmstead-era reasoning that intangible conversations could never be searched or seized.
Applied to the facts, Katz had a right to privacy when he stepped into the booth and shut the door behind him. What he sought to keep private, his conversation, was constitutionally protected even though the booth itself sat in a public space. Because the FBI conducted its surveillance without a warrant, the recordings were inadmissible.
The majority made one point that has shaped Fourth Amendment law ever since. The FBI’s surveillance was narrowly focused and probably could have been authorized by a warrant if agents had bothered to ask for one. The problem was not the scope of the surveillance. It was the total absence of judicial oversight. The amendment does not forgive that shortcut no matter how careful the investigation.
The Reasonable Expectation of Privacy Test
Justice John Harlan’s concurrence provided the specific test courts still use. Harlan proposed a two-part framework for deciding whether government conduct is a Fourth Amendment search.4Constitution Annotated. Amdt4.3.3 Katz and Reasonable Expectation of Privacy Test
The first part is subjective: did the person actually expect privacy? This turns on what the individual did, not just what they hoped for. Katz demonstrated his expectation by entering the booth and closing the door.
The second part is objective: is that expectation one society is prepared to recognize as reasonable? Someone whispering secrets in a packed restaurant would struggle to meet this standard, because the setting itself makes overhearing likely. A closed phone booth, even in public, created a space where most people would reasonably expect not to be recorded.5Legal Information Institute. Katz and the Adoption of the Reasonable Expectation of Privacy Test
Both prongs must be satisfied. When they are, law enforcement needs a warrant supported by probable cause and issued by a neutral judge or magistrate.6Legal Information Institute. Federal Rules of Criminal Procedure Rule 41 – Search and Seizure Evidence obtained without that warrant is generally excluded from trial under the exclusionary rule.7Constitution Annotated. Amdt4.7.1 Exclusionary Rule and Evidence
Justice Black’s Dissent
Justice Hugo Black was the lone dissenter. His objection was textual. The Fourth Amendment protects “persons, houses, papers, and effects,” and those words, in Black’s reading, referred to tangible, physical things. A conversation, he argued, could not be searched or seized under any natural reading of that language.
Black accused the majority of rewriting the Constitution to match modern preferences. He wrote that the amendment “simply does not apply to eavesdropping” and criticized the Court for using “clever word juggling” to stretch language aimed at physical intrusions. If the country wanted to protect phone conversations from government surveillance, he said, the proper route was a constitutional amendment, not judicial interpretation.
The Limit Built Into the Ruling
Katz carried a significant limitation. The majority noted that the Fourth Amendment does not protect information a person “knowingly exposes to the public.” That principle became the foundation of what courts call the third-party doctrine.
In Smith v. Maryland (1979), the Court held that dialing a phone number meant voluntarily handing that information to the phone company, so a device recording only the numbers dialed was not a search. In United States v. Miller (1976), the Court reached the same conclusion about checks and deposit slips given to a bank. Under this doctrine, information you share with a business in the ordinary course of dealing with them generally loses Fourth Amendment protection when the government asks the business for it.
How Katz Applies to Modern Surveillance
The reasonable expectation of privacy test has proven durable, but applying a 1967 framework to modern technology has forced the Court to revisit it repeatedly.
GPS Tracking
In United States v. Jones (2012), FBI agents attached a GPS tracker to the underside of a suspect’s car and monitored his movements for 28 days. The Court unanimously held this was a search, but the justices split on why. Justice Scalia’s majority opinion revived the older trespass test: physically placing a device on someone’s private property to gather information is a search regardless of privacy expectations. The opinion made clear that Katz’s privacy-based test was “added to, but not substituted for,” the property-based approach.8Legal Information Institute. United States v. Jones
Jones matters because it confirmed that both tests remain valid. The government can trigger the Fourth Amendment either by physically intruding on property or by violating a reasonable expectation of privacy.
Cell Phone Searches
In Riley v. California (2014), police searched an arrestee’s smartphone without a warrant, relying on the exception that lets officers inspect items found on someone they arrest. The Court unanimously rejected that reasoning. Digital information on a cell phone cannot be searched without a warrant simply because the phone was in an arrestee’s pocket.9Justia. Riley v. California, 573 U.S. 373 (2014) The Court recognized that a phone holds years of photos, messages, browsing history, and location data, an “intimate window” into someone’s entire life that no physical item in a pocket could match.
Cell-Site Location Data
Carpenter v. United States (2018) brought the tension between Katz and the third-party doctrine to a head. The FBI obtained 127 days of historical cell-site records showing everywhere Timothy Carpenter’s phone had connected to a tower. The government argued that no warrant was needed because Carpenter had voluntarily shared that data with his carrier.
The Court disagreed 5–4. Individuals retain a legitimate expectation of privacy in the record of their physical movements, even when a third party technically holds the data. The Court declined to extend the third-party doctrine to these “novel circumstances,” noting that cell-site records provide “an all-encompassing record of the holder’s whereabouts.”10Supreme Court of the United States. Carpenter v. United States, 585 U.S. 296 (2018) Carpenter did not overrule the third-party doctrine, but it made clear that automatic application of Smith and Miller to every kind of digital record is no longer safe.
Why the Case Still Matters
Before 1967, the Fourth Amendment functioned largely as a property rule. If the government stayed off your land and out of your home, it could surveil you however it liked. After Katz, the question became whether your expectation of privacy was reasonable, regardless of where you were standing or what technology the government used.
That shift has proven essential as surveillance has outpaced anything the 1967 Court could have imagined. Facial recognition, email monitoring, smart-home devices, and AI-driven data analysis all raise questions that trace back to Harlan’s two-part test. Each new technology forces courts to ask the same pair of questions: did the person expect privacy, and does society agree that expectation was reasonable? For nearly six decades, Katz has been the starting point for every Fourth Amendment challenge to government surveillance, and no court has proposed a workable replacement.