Presidential succession is referred to multiple times in the U.S. Constitution, as well as the Twentieth Amendment and Twenty-fifth Amendment. The vice president is the only officeholder named in the Constitution as a presidential successor. The Article II succession clause authorizes Congress to designate which federal officeholders would accede to the presidency in the event that the vice president were unable to do so, a situation which has never occurred. The current Presidential Succession Act was adopted in 1947 and last revised in 2006. The order of succession is as follows: the vice president, the speaker of the House of Representatives, the president pro tempore of the Senate, and then the eligible heads of the federal executive departments who form the president's Cabinet in the order of creation of the department, beginning with the secretary of state.