Definition of Exclusionary Rule
The exclusionary rule is derived from the Fourth Amendment which states that no person "shall be compelled in any criminal case to be a witness against himself" and that no person "shall be deprived of life, liberty or property without due process of law." The exclusionary rule does not allow evidence to be used in court if it was obtained illegally or without a proper search warrant. It is a legal principle that holds true under constitutional law.
The Exclusionary Rule was elaborated in the 1960s to cover other parts of law enforcement procedures, such as wiretapping evidence and involuntary confessions. There are exceptions and limitations to this rule. For instance, evidence may be used if a second, untainted source has a major role in finding the evidence, the evidence would have been discovered anyway without the tainted evidence or the evidence is to be used for impeaching a witness on cross examination. Additionally, a witness's identification of the defendant is not excluded if the witness could identify the defendant before an illegal arrest (witnesses recognizes the defendant from the crime, not from the line up), the evidence is for a grand jury proceeding, or state agents acted under a good faith belief that they were complying with the Fourth Amendment.