Fourth Amendment to the United States Constitution

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The Fourth Amendment (Amendment IV) to the United States Constitution is part of the Bill of Rights. It prohibits unreasonable searches and seizures and sets requirements for issuing warrants: warrants must be issued by a judge or magistrate, justified by probable cause, supported by oath or affirmation, and must particularly describe the place to be searched and the persons or things to be seized.

Fourth Amendment case law deals with three main issues: what government activities are "searches" and "seizures", what constitutes probable cause to conduct searches and seizures, and how to address violations of Fourth Amendment rights. Early court decisions limited the amendment's scope to physical intrusion of property or persons, but with Katz v. United States (1967), the Supreme Court held that its protections extend to intrusions on the privacy of individuals as well as to physical locations. A warrant is needed for most search and seizure activities, but the Court has carved out a series of exceptions for consent searches, motor vehicle searches, evidence in plain view, exigent circumstances, border searches, and other situations.

The exclusionary rule is one way the amendment is enforced. Established in Weeks v. United States (1914), this rule holds that evidence obtained as a result of 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". The exception is if it inevitably would have been discovered by legal means.

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 submitted the amendment to the states on September 28, 1789. By December 15, 1791, the necessary three-fourths of the states had ratified it. On March 1, 1792, Secretary of State Thomas Jefferson announced that it was officially part of the Constitution.

Because the Bill of Rights did not initially apply to state or local governments, 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 state and local governments in Mapp v. Ohio (1961) via the Due Process Clause of the Fourteenth Amendment.