What does the Fourth Amendment require?

The Fourth Amendment generally requires police to obtain a warrant supported by probable cause before conducting a search or seizure. The amendment's text, ratified on December 15, 1791, declares that 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. Probable cause is more than a hunch but less than the proof required to convict; the Supreme Court defined it in Brinegar v. United States (1949) as a reasonable belief that a crime has been committed and evidence will be found in the place to be searched.

The warrant must be issued by a neutral and detached magistrate, usually a judge or magistrate judge, who reviews a sworn affidavit from law enforcement before signing. The particularity requirement bars general warrants of the kind British colonial officers wielded as writs of assistance. A valid warrant must specify the address to be searched and the categories of evidence sought, such as a particular firearm, narcotics, or business records. Courts have invalidated warrants that simply authorized officers to look for evidence of crime without identifying the offense. Maryland v. Garrison (1987) addressed warrants that mistakenly described the wrong apartment, and Groh v. Ramirez (2004) struck down a warrant that listed no items at all.

Despite the warrant default, the Supreme Court has recognized many exceptions for situations where obtaining one is impractical or unnecessary. These include consent searches when the person voluntarily agrees, search incident to lawful arrest under Chimel v. California (1969), automobile searches under Carroll v. United States (1925) because of vehicle mobility, plain view seizures of contraband during a lawful presence, hot pursuit of a fleeing suspect, exigent circumstances such as imminent destruction of evidence, the protective sweep doctrine from Maryland v. Buie (1990), border searches, and stop and frisk based on reasonable suspicion under Terry v. Ohio (1968). Administrative inspections and certain regulatory searches in heavily regulated industries also fall outside the warrant rule.

Evidence obtained in violation of these requirements is generally excluded from criminal trials under the exclusionary rule announced in Weeks v. United States (1914) and applied to states in Mapp v. Ohio (1961), with a good faith exception recognized in United States v. Leon (1984) when officers reasonably rely on a defective warrant. Riley v. California (2014) required a warrant for cell phone contents, and Carpenter v. United States (2018) required one for historical cell tower data. Naturalization candidates should remember that the Fourth Amendment shields people, homes, phones, and effects from arbitrary search, with the warrant requirement as the central safeguard.