Judicial review works by validating or invalidating the constitutionality of legislative and executive acts of government. This special power was not delegated by the United States Constitution to the federal judiciary. It was only established by the U.S. Supreme Court in 1803.
In the landmark case of Marbury v. Madison, Chief Justice John Marshall ruled that if a law is in conflict with the Constitution, then the court must declare its unconstitutionality and nullify the law. His assertion was based on the premise that the Constitution is the supreme law of the land. Therefore, as final interpreters of the Constitution, it is the duty of the judicial branch to review congressional and executive decrees.