Katz v. United States and the Phone Booth That Made Privacy Constitutional

Charles Katz liked a particular bank of public telephones on Sunset Boulevard in Los Angeles. In the mid-1960s, he used them to place bets and pass along wagering information to people in other states, which federal law forbade. FBI agents noticed the routine. Rather than tap the phone line, they taped a microphone and recorder to the outside of the booths, switched it on when Katz walked in and recorded his side of the conversations. The agents never opened the door. They never touched the phone. By the government's reasoning, they had not searched anything at all. The case that grew out of those recordings reached the Supreme Court, and on December 18, 1967, the justices used a gambler's phone calls to rewrite what the Fourth Amendment means. Every argument about your cell phone, your email and your location history still runs through that glass booth.

In short: Before 1967, the Supreme Court treated the Fourth Amendment as a shield for physical places, so wiretaps without trespass needed no warrant. Katz v. United States changed that, holding that the Amendment protects people and their reasonable expectations of privacy. The ruling's strengths and gaps have shaped surveillance law ever since, all the way to the 2026 decision on Google location data.

The old rule: Olmstead and the property line

To see why Katz mattered, start forty years earlier. In 1928, in Olmstead v. United States, the Court considered the case of Roy Olmstead, a Seattle bootlegger whose phone lines had been tapped by federal agents during Prohibition. The taps were placed on wires outside his home and office. Chief Justice William Howard Taft, writing for a 5 to 4 majority, held that this was not a search. The Fourth Amendment, he reasoned, protected persons, houses, papers and effects against physical intrusion. Agents who listened to wires on public streets had not entered anything.

Justice Louis Brandeis wrote a dissent that became more famous than the majority. He warned that the government would find ever more subtle ways to invade privacy as technology advanced, and he described the founders' purpose as protecting a right to be let alone, which he called the most comprehensive of rights. Brandeis had co-written a law review article on the right to privacy in 1890, and in Olmstead he argued that the Constitution had to keep up with inventions the framers could not have imagined.

For four decades, the majority's property line governed. Courts asked whether agents had physically intruded into a protected area. A microphone pushed into a wall might be a search; a microphone resting against it might not. It was a test about carpentry, not privacy.

A booth, a microphone and a question of glass

Katz's lawyers and the government both argued the case within that old framework at first. The briefs debated whether a phone booth was a constitutionally protected area, and whether attaching a device to its outside counted as physical penetration. It was the kind of question that made sense under Olmstead and sounds absurd today.

The Court refused to play along. Justice Potter Stewart, writing for the majority, said the parties had framed the issue wrongly. Asking whether a phone booth was a protected area missed the point, because, as he put it, "the Fourth Amendment protects people, not places." What a person knowingly exposes to the public, even at home, is not protected. But what a person seeks to keep private, even in a place open to the public, may be.

Katz had shut the door of a booth and paid for a call. In doing so, he was entitled to assume that what he said into the mouthpiece would not be broadcast to the world. The government's eavesdropping violated the privacy on which he justifiably relied, and that was a search. Because the agents had not obtained a warrant, the recordings could not be used against him.

Stewart did not say the FBI was forbidden to listen. He noted that the agents had acted with restraint and probably could have gotten a warrant from a judge if they had asked. The constitutional problem was that they had not asked. A neutral magistrate, not the officers themselves, had to approve the search in advance.

The concurrence that became the test

The vote was 7 to 1, with Justice Thurgood Marshall not taking part. Justice Hugo Black dissented, arguing that the Fourth Amendment's words, about searches of things and seizures of things, could not be stretched to cover overheard conversations. He believed the Court was rewriting the Constitution to suit modern times, a charge that originalists still make about Katz.

But the opinion that lasted longest was a short concurrence by Justice John Marshall Harlan. He offered a two-part formula. First, has the person shown an actual, subjective expectation of privacy? Second, is that an expectation society is prepared to recognize as reasonable? If both, a government intrusion is a search.

Lower courts and later Supreme Court majorities adopted Harlan's formula as the standard. The "reasonable expectation of privacy" test became the central idea of Fourth Amendment law for the next half century, cited in thousands of cases involving cars, trash, backyards, bank records and electronic devices.

Black's objection, taken seriously

Justice Black's dissent is easy to wave away as a relic. It deserves better, because the argument he made is alive on today's Court and because answering it shows why Katz was right.

Black's point was about words. The Fourth Amendment protects "persons, houses, papers, and effects" against unreasonable "searches and seizures." A conversation, he argued, is not a thing that can be searched or seized in the ordinary sense. The framers knew about eavesdropping, which in the eighteenth century meant standing under the eaves of a house to listen at a window, and they chose not to mention it. If the Constitution was going to forbid electronic eavesdropping without a warrant, he wrote in substance, the people should amend it, or Congress should pass a law. Judges should not do it by reinterpretation.

There is a principle in that, and it is not a frivolous one. Judges who loosen constitutional text to reach results they like can loosen it in directions others will not like. Progressives have learned that lesson in recent years from rulings they disagree with.

The answer, in my view, is that Katz did not abandon the text so much as take its purpose seriously. The framers wrote the Fourth Amendment in reaction to general warrants and writs of assistance, which let British officers rummage through homes and businesses looking for anything incriminating. The evil they targeted was the government's power to pry into private life without a specific justification approved in advance. Microphones and wiretaps do the same thing more efficiently. Reading the Amendment to cover a physical rummage through a desk drawer but not a recording of every word spoken in a closed booth would honor the letter and defeat the point.

There is also a practical reply. Black was right that Congress could act, and it did, the next year. But Congress acted because the Court had made clear that the Constitution required something. Before Katz, decades of Olmstead had produced no comprehensive federal rule for wiretaps by federal agents. The constitutional ruling was what made the statute happen.

Congress answers within months

Katz set off quick action in Congress. In June 1968, President Lyndon Johnson signed the Omnibus Crime Control and Safe Streets Act, whose Title III created detailed rules for government wiretapping. Agents investigating listed crimes could intercept calls only with a court order, issued on probable cause, limited in time and scope, with a duty to minimize the capture of unrelated conversations. Unauthorized private wiretapping became a crime too.

Title III is an underappreciated example of courts and Congress working together. The Court announced a principle in Katz, and in Berger v. New York the same year it described what a constitutional eavesdropping law would need to look like. Lawmakers wrote a statute that met those standards. The result was a system of warrants that still governs wiretaps today, and the yearly wiretap reports that federal courts publish trace back to it.

Where the test went wrong

The reasonable-expectation test had a weakness that took years to show. If privacy depends on what society expects, then the more surveillance becomes normal, the less privacy the Constitution protects. The test can shrink as technology spreads.

The sharpest example was the third-party doctrine. In United States v. Miller in 1976, the Court held that people have no reasonable expectation of privacy in bank records, because they share that information with the bank. In Smith v. Maryland in 1979, it held the same about the phone numbers a person dials, which the phone company records. Information handed to a business, the reasoning went, had been knowingly exposed.

In a world of paper checks and landlines, the doctrine looked narrow. In a world where nearly everything people do creates a record held by a company, it threatened to swallow the Fourth Amendment. Your search history, your emails, your location data and your purchases all pass through third parties. Taken literally, Smith and Miller would let the government obtain all of it without a warrant.

There is a fair conservative critique here, and it partly overlaps with the progressive one. Some justices, including Justice Neil Gorsuch, have argued that Katz's test is too vague and that courts should return to property-based reasoning, treating data as a person's papers and effects even when a company holds it. Progressives tend to defend Katz. Both camps agree the third-party doctrine has gone too far.

From the phone booth to the phone in your pocket

The Court has spent the twenty-first century trying to apply Katz to new tools. In 2001, it held that pointing a thermal-imaging device at a home to detect heat inside was a search. In 2012, in a case about a GPS tracker attached to a car, it found a search based on the physical trespass of attaching the device, and several justices wrote that long-term tracking would violate reasonable expectations as well. In 2014, it held unanimously that police generally need a warrant to search the contents of a cell phone seized during an arrest.

In 2018, in Carpenter v. United States, the Court held that the government generally needs a warrant to obtain a person's historical cell-site location records from a phone company, declining to extend the third-party doctrine to that kind of data. And in June 2026, in Chatrie v. United States, the Court held that getting a person's cell-phone location data from Google is a Fourth Amendment search. Each of those rulings is, in a direct line, an answer to the question Stewart asked about the booth: what does a person reasonably expect to keep private?

There is a pleasing irony in where this ended up. Phone booths have nearly vanished from American streets. The device that replaced them goes everywhere its owner goes and records far more than any eavesdropper could have heard on Sunset Boulevard. The principle built for a glass box now protects, imperfectly, the most revealing object most people own.

Why the principle still needs defending

Katz did not settle the hard questions. It moved them. Courts still argue about whether data brokers can sell location information to agencies that would need a warrant to get it from a phone company. Intelligence programs operate under separate rules with less court review. And the test still tends to give way when new forms of surveillance become routine.

That is why the Fourth Amendment cannot be left only to judges. Title III came from Congress, and the strongest protections for digital data may have to come from Congress too, in the form of warrant requirements written into statute. A legislature can draw a line once and clearly, rather than waiting for a gambler, a bootlegger or a robbery suspect to bring the right case to court.

Charles Katz's legal fame rests on bets placed by phone. His case gave every American a stronger claim to privacy than the Court had recognized before. That is how civil liberties often grow in this country: through cases brought by people no one would choose as heroes, decided on principles that protect everyone.

What you can do

Privacy rights grow when courts, Congress and citizens all push. Some practical places to start:

1. Follow the Court. Court Watch lists the Supreme Court's opinions this term with the Court's own summaries, along with the emergency docket and Senate votes on judges, so you can see the next Fourth Amendment ruling as soon as it comes down.
2. Tell Congress to write warrant rules into law. One address in find your representatives brings up everyone who speaks for you in Washington and the statehouse, with numbers to call, forms to write through and their recent roll calls. Ask whether they support requiring warrants before agencies buy or search Americans' data.
3. Read the wider story. The democracy and the rule of law topic page follows surveillance, the courts and limits on government power.
4. Back the lawyers. Civil liberties and legal aid groups bring many of these cases. The organizations directory lists nonprofits by cause in every state, including civil rights and legal aid groups near you.