The Fourth Amendment reads: "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."

This protects the privacy of the home against government intrusion. It applies to criminal investigations. Unlike the Fifth and Sixth amendments, nothing in its text restricts it to criminal cases.

The Fourth Amendment originated as a ban on the British device called the "general warrant," a warrant that allowed the seizure of anything in the suspect's home. Under the Amendment, a search requires a specific warrant. Even a search supported by a warrant will be found unconstitutional by the courts, unless the warrant was issued by a neutral magistrate, such as a judge or a justice of the peace, and unless it is limited to a certain place and certain things to be found.

To properly limit the Fourth Amendment, the Supreme Court has adopted the "exclusionary rule." Any evidence gathered in a way that violates the intent of the Fourth Amendment may not be used at trial. An example of this rule in practice would be a warrant to allow police to search an apartment for a stolen stereo set. During the search, drug paraphernalia is discovered in a dresser drawer. The discovery of this evidence of drug use would be inadmissible in a subsequent trial although it could be used as the basis for obtaining yet another search warrant, this time for drugs. The exclusionary rule has been criticized because often criminals are released due to police errors irrespective of their apparent guilt. If the evidence used to convict them was wrongfully obtained, the criminal walks.

Defenders of the rule, however, argue that the police have adapted their procedures to conform with it and anything less would allow police to evade the specific intent of the Fourth Amendment. The long-standing and ongoing debate was partly defused by the Supreme Court in 1984 when it conceded an exception to the warrant requirement if police can demonstrate to the satisfaction of the court that they acted in "good faith" on a warrant that later proved to be defective.

An emerging issue is the application of the Fourth Amendment to searches made without a warrant by social workers in child abuse investigations. In a case now being litigated in California by the Home School Legal Defense Association, the state is arguing, in effect, that while police have to observe the Fourth Amendment, social workers, who are not law-enforcement agents, do not. HSLDA replies that the amendment protects the "security" of "persons" in their "houses" regardless of whether the would-be searchers are law-enforcement agents or not.