The Fourth Amendment

The Fourth Amendment protects the right of the people to be secure in their persons, houses, papers, and effects against unreasonable searches and seizures, and requires that warrants issue only upon probable cause, supported by oath, describing the place to be searched and the persons or things to be seized. It grew out of colonial anger at general warrants and writs of assistance, which let British officials search homes and businesses at will for smuggled goods.

Courts decide whether a search was reasonable, and warrants are generally required for homes, though many exceptions exist, such as consent, searches incident to arrest, and emergencies. In Mapp v. Ohio in 1961, the Supreme Court applied the exclusionary rule, which bars the use of illegally obtained evidence, to the states. Katz v. United States in 1967 held that the amendment protects people's reasonable expectations of privacy, not just places, and Terry v. Ohio in 1968 allowed police to stop and frisk people on reasonable suspicion.

Technology has made the Fourth Amendment central to modern government. In Riley v. California in 2014, the Court required a warrant to search a cell phone seized during an arrest, and in Carpenter v. United States in 2018 it required a warrant for historical cell phone location records. Foreign intelligence surveillance is governed by a separate statutory system overseen by a secret court, and debates continue over government purchases of commercial data, facial recognition, and border searches of electronic devices.