Mapp v. Ohio (1961) strengthened the Fourth Amendment protection against unreasonable searches and seizures, making it illegal for evidence obtained without a warrant to be used in a criminal trial in state court. This 5-4 decision is one of several cases decided by the Warren Court in the 1960s that dramatically expanded the rights of criminal defendants.

When Cleveland police searched Dollree Mapp’s home for a suspect in a local bombing, she refused to let them in without a search warrant. A few hours later, more officers arrived and forced their way into Mapp’s home. When one officer held up a piece of paper that he said was a warrant, Ms. Mapp snatched it but lost it in a struggle with the officer. In the course of their search, the police found allegedly obscene literature and photos that Mapp said were not hers. Although no warrant was produced at her trial, she was convicted of possessing pornography. Mapp’s attorney argued that Ohio’s obscenity law was unconstitutionally vague, but it was the ACLU’s “friend of the court” brief that called for the exclusion of evidence seized illegally. In his opinion for the Court, Justice Tom Clark wrote that because it was seized without a warrant, the evidence could not be used in Mapp’s trial. Previously, illegally seized evidence had only been excluded in federal criminal trials. This was the first in a series of decisions impacting protections for criminal defendants.