One of the cornerstones of US constitutional protections is the exclusionary rule. The exclusionary rule states that evidence obtained as a result of an illegal search or seizure is not admissible against the victim of the illegal search in a criminal proceeding. The rationale behind this rule is that if the government cannot use the evidence in a criminal trial, it will be less likely to conduct illegal search and seizures in the future.
The exclusionary rule was applied in Federal Courts in Weeks. v. United States, 232 U.S. 383 (1914). The principal was later incorporated against the states in Mapp v. Ohio, 367 U.S. 643 (1961). This rule applies regardless of whether the violation is conducted by a state or federal official and regardless of whether the criminal prosecution is brought in a state or federal court.
While many other countries have some version of the exclusionary rule, the United States is unique that it is the only country in which the exclusionary rule is mandatory.
Example: Baltimore Police break into defendants home without a warrant, finding a small amount of marijuana in the defendant's sock drawer. The evidence will be inadmissible because the evidence was found pursuant to an illegal search in violation of the defendant's Fourth Amendment Rights.
The United States Supreme Court has slowly been eroding several cornerstones of the exclusionary rule and many academics now consider the rule to be in danger. The following exceptions to the rule generally apply:
Other Uses - Evidence which would otherwise be inadmissible at trial may be used for other purposes such as grant jury proceedings, bail hearings and other preliminary matters as well as sentencing and parole hearings.
Civil Suits - Generally, the exclusionary rule only applies in criminal proceedings. Therefore, the evidence may be admissible in a civil suit against the defendant for damages.