The Exclusionary Rule is a legal principle aimed at protecting the accused right to Constitutional protection under the Fourth, Fifth, and Sixth Amendments. Prior to the implementation of the rule, judges could allow any evidence they felt reasonably dealt with the issue at hand. In Weeks v. United States (1914) the US Supreme Court first utilized the rule to overturn a conviction based upon a warrantless search. It was delegated outside of federal court in 1961 via Mapp v. Ohio.
There are two justifications for the implementation of the rule. First, it is the theoretical foundation that states the government of the people should refrain from violating the Constitution or engaging in tyrannical behavior toward the citizens. The second, more factual argument is the exclusion of evidence in a case will prevent law enforcement from engaging in actions that would weaken their investigation. The Supreme Court explained, "Its purpose is to deter."
There are exceptions to the rule. The "Good Faith" doctrine could allow evidence seized in violation of the Constitution if the person seizing it did under the true auspice of legality. The concept of inevitable discovery may also lead to the inclusion of the evidence if the government can argue the evidence would have been discovered in the course of a normal investigation.
The rule is designed to promote a citizen's Constitutional rights by punishing the government for violating one of the amendments protecting a person against unreasonable government actions as it pertains to criminal cases.