Because almost all defendants charged with crimes are in fact guilty, they have few legitimate ways to defend against the charges. Defendants' first line of defense in a great many cases is to try to suppress the evidence of their guilt. To do that, they have to find some misstep by police that would require the court to exclude incriminating evidence.
There are various exclusionary rules under the Fourth, Fifth, Sixth, and Fourteenth Amendments, but the one people generally have in mind when they use the term "exclusionary rule" is the one that applies to evidence obtained by a search or seizure that doesn't comply with the Fourth Amendment case law. This exclusionary rule has applied in federal court since 1914 (Weeks v. U.S.), and to all of the states since 1961. (Mapp v. Ohio) The basic rule is that evidence derived from an unreasonable search or seizure is generally inadmissible against a defendant whose Fourth Amendment rights were violated by that search or seizure.









