The Exclusionary Rule was designed to give police an incentive to avoid violating the 4th Amendment.  This rule is controversial because if the evidence is found to be improperly gathered, it is excluded, and the suspect cannot be convicted.  The major argument for The Exclusionary Rule is that law-abiding citizens have a stake in police officers doing their job correctly even if it means that sometimes a criminal goes free.  

The King v. Jane Warickshall (1783) is the starting point for the 4th Amendment Exclusionary Rule.  In the case Jane Warickshall, a British Citizen, confessed that she had received stolen goods from Thomas Littlepage.  Consequently, her room was searched and the stolen property was found under her bed.  Warickshall appealed the verdict because she was coerced through fear to confess.  The court found that while Warickshall's testimony was inadmissible, the property discovered under the bed was admissible in court.  This court case was highly controversial and was a highly debated subject during the creation of the Bill of Rights.  The Exclusionary Rule in the 4th Amendment is tied to this case.  

In 1886, The Exclusionary Rule was refined.  The case Boyd v. United States determined that the Fourth Amendment of the United States Constitution protects against the invasion into a person’s private matters even in writing.  This decision was weak in that it did not exclude all evidence that was improperly gathered under the 4th Amendment, but instead excluded only testimony that was self-incriminating.  Verbal self-incrimination was already protected through the 5th Amendment; however, Boyd expanded this to include documents such as letters, ledgers, etc. whereby a person would provide self-incriminating evidence by handing them over to the courts via a subpoena.  

The strong version of The Exclusionary Rule was announced through the case Weeks v. United States.  In the case, the police entered Week's home and searched his residence without a warrant.  The search found that Weeks had been transporting lottery tickets through the mail.  The case was appealed under the issue of whether Weeks' right to privacy and protection from unreasonable search and seizure had been violated. The Supreme Court found that there had been a violation of Weeks 4th Amendment rights and that the Federal Government did not have the right to enter and search a citizen's residence without a warrant.  Because of this, all evidence found during the illegal search had to be excluded from the court proceedings.  While Weeks affected Federal search and seizure, it lacked the jurisdiction to effect State search and seizure.  

In 1920, the Exclusion Rule's "fruit of the poisonous tree" doctrine was added.  This happened through the case of Silverthorne Lumber Co v. United States.   This case involved the two owners of the Silverthorne Lumber Co.  Both were arrested and while they were in jail, federal officers went to their office and collected evidence without a warrant.  The officers then used the information collected to investigate the case further.  Silverthorne Lumber Co v. United States determined that not only was evidence collected without a warrant inadmissible in court, but also any evidence collected due to information gathered from evidence seized in a warrant-less search also inadmissible in court.  This exclusionary rule has become known as "The Fruit of the Poisonous Tree."  

Wolf v. Colorado, in 1949, ruled that states were not required to adopt The Exclusionary Rule established through Weeks and Silverthorne Lumber Co.  While many states had adopted the rules established in these court cases, several of the states had significant limitations and exclusions.  

Mapp v. Ohio (1961) made The Exclusionary Rule as defined by Weeks and Silverthorne Lumber Co. the law of the land.  The case: Mapp v. Ohio involved the police approaching the residence of Dollree Mapp looking for a bombing suspect.  Mapp refused to give the officers entry to the residence.  She demanded a search warrant.  The officers remained at the residence for three hours.  The standoff ended as the police entered the residence showing Mapp a piece of paper they claimed was a warrant.  Mapp grabbed the paper and stuffed it down her blouse.  The police handcuffed her, retrieved the paper (which was "lost" shortly after) and searched the house.  During the search, they found several nude drawings and erotic books.  Mapp was arrested for possessing obscene materials.  The Supreme Court, after reviewing the circumstances of the case, determined that the evidence was inadmissible because the search was illegal under The Exclusionary Rule of the 4th Amendment.   

In the decision, Justice Tom Clark stated that it was illogical to require The Exclusionary Rule for the Federal Government and not the State Governments.

Moreover, our holding that The Exclusionary Rule is an essential part of both the Fourth and Fourteenth Amendments is not only the logical dictate of prior cases, but it also makes very good sense.  There is no war between the Constitution and common sense.  Presently a federal prosecutor may make no use of evidence illegally seized, but a state's attorney across the street may... Thus the state by admitting evidence unlawfully seized, serves to encourage disobedience to the federal Constitution, which it is bound to uphold.  

While The Exclusionary Rule is considered the “Law of the Land,” it does have exceptions.  In the Supreme Court Case, United States v. Leon (1984) the SCOTUS ruled that if police officers believe that the search warrant they executing is valid, and they are acting in "good faith" the evidence can be admitted even if there is a technical problem with the warrant.

 Nix v. Williams (1984) provided the exception of "inevitable discovery."  This states that evidence illegally collected under the 4th Amendment can be admissible in court, if the police can prove that they would have discovered it independently of the illegal search.  

There have been two major justifications for the exclusionary rule during the time that it has existed.

First, courts have justified the rule by saying that it is necessary to uphold the 4th Amendment.  This justification was first presented by the Supreme Court in Weeks v. US in 1914.  In this justification, the rationale is that people have a constitutional right to be free of illegal searches and seizures.  The exclusionary rule is the way to enforce this right.

More recently, a different justification has arisen.  This is the deterrence justification.  It says that the purpose of the exclusionary rule is to deter police misconduct.  The rationale here is that police will not conduct illegal searches or seizures if they cannot get any benefit from that action.  If the illegal acts do not help them gain convictions, they will not commit those acts.  This is a more utilitarian justification while the previous justification makes the exclusionary rule an integral part of the 4th Amendment.