In United States constitutional law, the exclusionary rule is a legal principle holding that evidence collected or analyzed in violation of the U.S. Constitution is not admissible for a criminal prosecution in a court of law (that is, it cannot be used in a criminal trial). The rule is designed to provide a remedy and disincentive, short of criminal prosecution, for prosecutors and police who illegally gather evidence in violation of the Fourth and Fifth Amendments in the Bill of Rights, which provide for protection from unreasonable searches and seizure and compelled self-incrimination.

The exclusionary rule was created in the case of Weeks v. United States. This decision, however, created the rule only on the federal level. It was not until Mapp v. Ohio, 367 U.S. 643 (1961) that the exclusionary rule was also binding on the states through the Fourteenth Amendment, which guarantees due process. This landmark decision was considered by former Justice Potter Stewart as "the most important search and seizure decision in [American] history."

The exclusionary rule applies to all citizens or aliens (illegal or documented) that reside within the United States. It is not applicable to aliens residing outside of U.S. borders. United States v. Alvarez decided that property owned by aliens in a foreign country is admissible in court. Certain persons in the U.S. receive limited protections, such as prisoners, probationers, parolees, and persons crossing U.S. borders. Corporations, by virtue of being, also have limited rights under the Fourth Amendment (see corporate personhood).

The fruit of the poisonous tree doctrine holds that any evidence discovered indirectly through an illegal search or seizure is inadmissible in court. The rationale is that the evidence would not have been found were it not for the violation of the Fourth Amendment and therefore must be suppressed because it is the "fruit of the poisonous tree."

http://en.wikipedia.org/wiki/Exclusionary_rule