The purpose of the Exclusionary Rule is to prevent the authorities from using evidence obtained in violation of the United States Constitution.

The Exclusionary Rule was created to protect criminal suspects by preventing them from being convicted on the basis of evidence obtained from illegal searches and seizures and that thus violated their constitutional rights.

Though the Exclusionary Rule is grounded in the Fourth Amendment to the Constitution, it wasn't until the Supreme Court expanded the Exclusionary Rule to state as well as federal authorities in Mapp v. Ohio (1961) that its radical implications became clear.

Initially, the Exclusionary Rule was supposed to protect citizens from overmighty federal authorities, but the Supreme Court's ruling in Mapp meant that, from then on, citizens would enjoy the protection of the Rule from overmighty state authorities too.

The Exclusionary Rule is traditionally applied in cases where law enforcement agencies, either at the state or federal level, illegally obtain evidence in a search or seizure that then forms the basis of a criminal charge. Under the Exclusionary Rule, any such evidence is tainted and therefore cannot be admissible in a court of law. In other words, it must be excluded from the prosecution's case.