A law is unconstitutional when it conflicts with the Constitution of that country. In the United States, generally federal courts are responsible for determining whether a law conflicts with the United States Constitution with the Supreme Court of the United States being the final arbiter. Courts have had this power - called judicial review - since Marbury v. Madison although there is little explicit basis for the power in the Constitution itself.
Beginning in the last half of the 20th century, courts started to declare laws unconstitutional based on reasons and cases not contained in the Constitution itself.[Citation Needed]