Probable Cause, Warrants and Their Exceptions

The Fourth Amendment has two parts. The first protects the people against unreasonable searches and seizures of their persons, houses, papers and effects. The second requires that warrants be issued only upon probable cause, supported by oath, and particularly describing the place to be searched and the persons or things to be seized. Its authors had in mind general warrants and writs of assistance, and the English case Entick v. Carrington in 1765, which condemned an open-ended search of a writer's papers.

Courts generally treat warrantless searches as unreasonable, but they have recognized exceptions: consent, searches incident to a lawful arrest, items in plain view, emergencies, and the 'automobile exception' from Carroll v. United States in 1925, which permits searching a vehicle when officers have probable cause. Supporters say these exceptions reflect practical policing needs; critics say they have grown so numerous that warrants have become the exception rather than the rule.