Ker V. California

In Ker v. California, 374 U.S. 23 (1963), the U.S. Supreme Court held that State procedures for seizing evidence must conform to federal procedures as defined by judicial interpretations of the Fourth Amendment. Previously State criminal procedures for searches and seizures need only comply with the more flexible requirements of due process under the Fourteenth Amendment, rather than strict and complex federal interpretation of the Fourth Amendment.

The Court expanded this power in a case in which it affirmed a state court decision. This is often how the U.S. Supreme Court has taken new authority, by declaring it has the new power but not using it in the decision in which it claims the authority. See, e.g., Marbury v. Madison, Everson v. Board of Education.

As Justice John Harlan II later sought to overrule both "Mapp v. Ohio, 367 U.S. 643 (1961), and Ker v. California, 374 U.S. 23 (1963). The former of these cases made the federal 'exclusionary rule' applicable to the States. The latter forced the States to follow all the ins and outs of this Court's Fourth Amendment decisions, handed down in federal cases." Coolidge v. N.H., 403 U.S. 443, 490 (1971) (Harlan, J., concurring).