The Exclusionary rule 

At common law, all evidence, no matter how seized, could be admitted in court. In Weeks v. United States, 232 U.S. 383 (1914), however, the Supreme Court adopted the "exclusionary rule," under which evidence seized unlawfully is rendered inadmissible in court. The rule was made applicable to the states in Mapp v. Ohio 367 U.S. 643 (1961).

The exclusionary rule serves primarily to deter police officers from conducting unlawful searches and seizures. It has, however, a number of exceptions. In United States v. Leon, 468 U.S. 897] (1984), the Supreme Court applied the "good faith" rule and held that evidence seized by officers objectively and in good faith relying on a warrant later found to be defective was still admissible. The evidence would still be excluded if an officer dishonestly or recklessly prepares an affidavit forming the basis of the warrant, if the issuing magistrate abandons his neutrality, or if the warrant lacks sufficient particularity. It remains unclear whether the "good faith" exception applies to warrantless seizures. On January 8, 1974, the Supreme Court ruled that grand juries may use illegally obtained evidence in questioning witnesses. United States v. Calandra, 414 U.S. 338.

The Supreme Court has held the rule does not apply in certain situations: (1) probation or parole revocation hearings; (2) tax hearings; (3) deportation hearings; (4) when government officials illegally seize evidence outside the United States; (5) when a "private actor" (i.e., not a State employee) illegally seized the evidence; or (6) when the illegally seized evidence is used to impeach the defendant's testimony. Furthermore, a defendant has standing to object to the admission of unconstitutionally seized evidence only if such seizure violated his own Fourth Amendment rights; a defendant may not assert the rights of a third party.

Closely related to the exclusionary rule is the "fruit of the poisonous tree" doctrine, under which the government is prohibited from introducing any evidence that was obtained subsequent to and as a result of the illegal search.

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