The exclusionary rule is a principle that holds that evidence seized in violation of the Fourth Amendment, which prohibits unreasonable searches and seizures, cannot be used to convict a defendant. In other words, the exclusionary rule excludes evidence that is illegally obtained. The famous saying attributed to Justice Benjamin N. Cardozo explained the exclusionary rule as “the criminal goes free because the constable has blundered.” It was (and is) considered controversial because it sometimes means that a guilty person can go free.

The Burger Court limited the exclusionary rule in its 1984 decision U.S. v. Leon by holding that the rule would not apply when a police officer acted in reasonable good faith on the validity of a search warrant. The good faith exception provides that “evidence need not be suppressed when police obtain the evidence through objective good faith reliance on a facially valid warrant that later is found to lack probable cause.” Under the Fourth Amendment, the police need probable cause to obtain a search warrant. The good-faith exception means that sometimes even if the search warrant was not backed by probable cause, the exclusionary rule will not apply and the evidence obtained through execution of the search warrant can be used at trial. The Court did not eliminate the exclusionary rule. The Leon exception applied to some search warrants that were found lacking in probable cause. The Court noted several exceptions to the good-faith exception, including: (1) where the magistrate issued the warrant based on a deliberately or recklessly false affidavit; (2) where the issuing magistrate failed to act in a neutral and detached manner; (3) where a warrant is based on an affidavit so lacking in evidence of probable cause as to render official belief in its existence entirely unreasonable; and (4) where a warrant is so devoid of information that the executing officers cannot reasonably presume it to be valid.