The Fourth Amendment, introduced to the Bill of Rights by James Madison, protects individuals against unreasonable search and seizure. These rights seek to balance the privacy interests of an individual against the law enforcement interests of the government. Lawful search and seizure therefore requires a warrant in most cases, with exceptions made for probable cause or emergency situations. The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and 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. – The Fourth Amendment to the United States Constitution

Subsequent rulings by the Supreme Court of the United States have defined the amendment’s protection to include a “reasonable expectation of privacy” and the exclusionary rule, which prohibits evidence obtained in an illegal search and seizure from being used in a court of law.

As police progress through stages of investigation, search incident to arrest, post-arrest, and post-conviction, the Court has considered different sets of circumstances in which to apply the Fourth Amendment. The law is not universally applied, but rather depends on context.

The Supreme Court under Chief Justice John G. Roberts, Jr., has varied its Fourth Amendment decisions since 2005. While the Roberts Court has tended to increase the scope of individual privacy, it has also indicated a strong likelihood to rule in favor of law enforcement and the state once a suspect has already been arrested.