One way courts enforce the Fourth Amendment is through the use of the exclusionary rule. The rule provides that evidence obtained through a violation of the Fourth Amendment is generally not admissible by the prosecution during the defendant's criminal trial
The Court adopted the exclusionary rule in Weeks v. United States (1914), prior to which all evidence, no matter how seized, could be admitted in court. In Silverthorne Lumber Co. v. United States (1920) and Nardone v. United States (1939), the Court ruled that leads or other evidence resulting from illegally obtained evidence are also inadmissible in trials.
The exclusionary rule and its effectiveness have often been controversial, particularly since its 1961 application to state proceedings. Critics charge that the rule hampers police investigation and can result in freeing guilty parties convicted on reliable evidence; other critics state that the rule has not been successful in deterring illegal police searches. Proponents argue that the number of criminal convictions overturned under the rule has been minimal and that no other effective mechanism exists to enforce the Fourth Amendment.
Responding to criticisms of the exclusionary rule, the Supreme Court has instituted numerous limitations to the rule. In United States v. Calandra (1974), the Court ruled that in questioning witnesses, grand juries may use evidence that allegedly was illegally obtained because, "the damage to that institution from the unprecedented extension of the exclusionary rule outweighs the benefit of any possible incremental deterrent effect."
The rule has also been held not to apply in the following circumstances:
evidence illegally seized by a "private actor" (i.e., not a governmental employee)
evidence seized from a common carrier
evidence collected by U.S. Customs agents
military discharge proceedings
evidence seized by probation or parole officers
child protective proceedings...