FBI agents electronically eavesdropped on Charles Katz's calls from a public phone booth without a warrant. The Court held the recording was a Fourth Amendment 'search': 'the Fourth Amendment protects people, not places.' Justice Harlan's concurrence articulated the enduring two-part test: a subjective expectation of privacy that society recognizes as reasonable. Katz is the doctrinal foundation of modern surveillance and digital-privacy law.