Fourth Amendment, amendment (1791) to the Constitution of the United States, part of the Bill of Rights, that forbids unreasonable searches and seizures of individuals and property. For the text of the Fourth Amendment, see below.

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Introduced in 1789, what became the Fourth Amendment struck at the heart of a matter central to the early American experience: the principle that, within reason, “Every man’s house is his castle,” and that any citizen may fall into the category of the criminally accused and ought to be provided protections accordingly. In U.S. constitutional law, the Fourth Amendment is the foundation of criminal law jurisprudence, articulating both the rights of persons and the responsibilities of law-enforcement officials. The balance between these two forces has undergone considerable public, political, and judicial debate. Are the amendment’s two clauses meant to be applied independently or taken as a whole? Is the expectation of privacy diminished depending on where and what is suspected, sought, and seized? What constitutes an “unreasonable” search and seizure?

The protections contained in the amendment have been determined less on the basis of what the Constitution says than according to what it has been interpreted to mean, and, as such, its constitutional meaning has inherently been fluid. The protections granted by the U.S. Supreme Court have expanded during periods when the court was dominated by liberals (e.g., during the tenure of Chief Justice Earl Warren [1953–69]), beginning particularly with Mapp v. Ohio (1961), in which the court extended the exclusionary rule to all criminal proceedings; by contrast, during the tenure of the conservative William Rehnquist (1986–2005) as chief justice, the court contracted the rights afforded to the criminally accused, allowing law-enforcement officials latitude to search in instances when they reasonably believed that the property in question harboured presumably dangerous persons.

The full text of the amendment is:

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