The Fourth Amendment (Amendment IV) is an amendment to the United States Constitution and part of the Bill of Rights. It prohibits unreasonable searches and seizures and requires any warrant to be judicially sanctioned and supported by probable cause. It was adopted in response to the abuse of the writ of assistance, a type of general search warrant issued by the British Government and a major source of tension in pre-Revolutionary America. The Fourth Amendment was introduced in Congress in 1789 by James Madison, along with the other amendments in the Bill of Rights, in response to Anti-Federalist objections to the new Constitution. Congress proposed the amendment to the states on September 28, 1789, and by December 15, 1791, the necessary three-quarters of the states had ratified it. Secretary of State Thomas Jefferson announced the adoption of the amendment on March 1, 1792.
Because the Bill of Rights did not initially apply to the states, and federal criminal investigations were less common in the first century of the nation's history, there is little significant case law for the Fourth Amendment before the 20th century. The amendment was held to apply to the states in Wolf v. Colorado (1949).
Under the Fourth Amendment, search and seizure (including arrest) should be limited in scope according to specific information supplied to the issuing court, usually by a law enforcement officer who has sworn by it. Fourth Amendment case law deals with three central questions: what government activities constitute "search" and "seizure"; what constitutes probable cause for these actions; and how violations of Fourth Amendment rights should be addressed. Early court decisions limited the amendment's scope to a law enforcement officer's physical intrusion onto private property, but with Katz v. United States (1967), the US Supreme Court held that its protections extended to the privacy of individuals as well as physical locations. Law enforcement requires a warrant for most search and seizure activities, but the Court has defined a series of exceptions for consent searches, motor vehicle searches, evidence in plain view, exigent circumstances, border searches, and other situations.
The amendment is enforced by the exclusionary rule. Established by Weeks v. United States (1914), this rule holds that evidence obtained through a Fourth Amendment violation is generally inadmissible at criminal trials. Evidence discovered as a later result of an illegal search may also be inadmissible as "fruit of the poisonous tree" unless it would have inevitably been discovered by legal means.