Katz v. United States
United States Supreme Court
The Fourth Amendment protects people, not places. A person has a reasonable expectation of privacy in a public telephone booth, and government electronic surveillance without a warrant violates the Fourth Amendment.
Case Summary
Katz v. United States established the foundational "reasonable expectation of privacy" test that governs Fourth Amendment analysis to this day. The FBI attached a listening device to the outside of a public phone booth used by Charles Katz to place illegal gambling wagers. The Supreme Court held this constituted a search under the Fourth Amendment, overturning the prior property-based trespass doctrine.
Background
Prior to Katz, Fourth Amendment protection was tied to physical trespass into a "constitutionally protected area." The FBI argued that because the listening device was placed on the outside of the phone booth — never physically entering it — no search occurred. The Supreme Court rejected this reasoning, shifting the constitutional inquiry to whether the individual had a reasonable expectation of privacy.
Why It Matters
Katz fundamentally reshaped American privacy law and has become even more significant in the digital age. The decision's framework — asking whether a person has a subjective expectation of privacy that society recognizes as reasonable — now governs questions involving cell phone tracking, email surveillance, digital data, and emerging technologies.