What is the Fourth Amendment?

The Fourth Amendment protects the people against unreasonable searches and seizures by government officials and generally requires that warrants be supported by probable cause. Ratified on December 15, 1791, its forty-five words read: The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.

The framers drew on bitter colonial experience with British writs of assistance, general search warrants that allowed customs officers to enter any home or business looking for smuggled goods. James Otis denounced them in his 1761 Boston speech, calling them the worst instrument of arbitrary power, and John Adams later credited that argument with helping to spark the American Revolution. James Madison drafted the Fourth Amendment to forbid such practices forever.

The Supreme Court has built a large body of doctrine around two questions. First, what counts as a search or seizure. Katz v. United States (1967) established that the amendment protects people, not places, and applies wherever a person has a reasonable expectation of privacy, including a public phone booth. Second, what makes a search reasonable. The default rule, articulated in cases including Mapp v. Ohio (1961) which extended the exclusionary rule to the states, requires a warrant issued by a neutral magistrate, supported by probable cause, and describing with particularity the place to be searched and the things to be seized.

Numerous exceptions allow warrantless searches: incident to lawful arrest under Chimel v. California (1969), automobile searches under Carroll v. United States (1925), exigent circumstances, plain view, consent, hot pursuit, border searches, administrative inspections, and stop and frisk based on reasonable suspicion under Terry v. Ohio (1968). The exclusionary rule from Weeks v. United States (1914) and Mapp v. Ohio (1961) generally bars prosecutors from using illegally obtained evidence, although United States v. Leon (1984) created a good faith exception.

Modern technology has reshaped the doctrine. Riley v. California (2014) required a warrant to search the contents of a cell phone seized during arrest, and Carpenter v. United States (2018) extended Fourth Amendment protection to historical cell site location records. Drone surveillance, license plate readers, and facial recognition continue to test the limits. Naturalization candidates should understand that the Fourth Amendment is the constitutional shield against arbitrary government intrusion into homes, vehicles, phones, and persons.