Like the rest of the Bill of Rights, the Fourth Amendment originally only applied in federal court. However, in Wolf v. Colorado, 338 U.S. 25 (1949), the U.S. Supreme Court ruled that the rights guaranteed by the text of the Fourth Amendment (sans the exclusionary rule to be discussed below) apply equally in state courts via the Fourteenth Amendment, which guarantees to the citizen of every state the right to due process and equal protection of the laws. The process by which the Supreme Court has made certain fundamental liberties protected by the Bill of Rights applicable to the states is known as the doctrine of incorporation.

Not every search and seizure that is scrutinized in state and federal court raises a Fourth Amendment issue. The Fourth Amendment only protects against searches and seizures conducted by the government or pursuant to governmental direction. Surveillance and investigatory actions taken by strictly private persons, such as private investigators, suspicious spouses, or nosey neighbors, are not governed by the Fourth Amendment. However, Fourth Amendment concerns do arise when those same actions are taken by a law enforcement official or a private person working in conjunction with law enforcement.

The Fourth Amendment does not apply even against governmental action unless defendants first establish that they had a reasonable expectation of privacy in the place to be searched or the thing to be seized. The Supreme Court has explained that what "a person knowingly exposes to the public, even in his own home or office, is not a subject of Fourth Amendment protection ... " But what he seeks to preserve as private, even in an area accessible to the public, may be constitutionally protected (see Katz v. United States, 389 U.S. 347 [1967]).