Mapp v. Ohio

According to the exclusionary rule governing search and seizure, evidence obtained illegally cannot be used to convict a suspect in a criminal case. There are a number of exceptions to the exclusionary rule, not least of which is the "good faith" exception. Thus when officers conduct a search that turns out to be illegal but which they believed in good faith to be legal while they were doing it--e.g., with an expired warrant, as in United States v. Leon (1984)--the evidence is still permissible. Illegally obtained evidence may be used to impeach the testimony of a defendant who testifies in their own defense; and a private citizen may use illegally obtained evidence, as long as he or she did not obtain it on orders from law-enforcement personnel. Moreover, the exclusionary rule applies only to criminal trials: evidence obtained illegally is not forbidden in civil cases, or in grand jury proceedings.