There have been 48 Vice Presidents of the United States since the office came into existence in 1789. Originally, the Vice President was the person who received the second most votes for President of the United States in the Electoral College. However, in the election of 1800, a tie in the electoral college between Thomas Jefferson and Aaron Burr led to the selection of the President by the House of Representatives. To prevent such an event from happening again, the Twelfth Amendment was added to the Constitution, creating the current system where electors cast a separate ballot for the vice presidency.
The United States Constitution assigns few powers or duties to the vice president. The Vice President's primary function is to succeed to the presidency if the President dies, resigns, or is impeached and removed from office. Nine vice presidents have ascended to the presidency in this way: eight (John Tyler, Millard Fillmore, Andrew Johnson, Chester A. Arthur, Theodore Roosevelt, Calvin Coolidge, Harry S. Truman and Lyndon B. Johnson) through the president's death, and one (Gerald Ford) through the president's resignation. In addition, the Vice President serves as the President of the Senate and may choose to cast a tie-breaking vote on decisions made by the Senate. Vice presidents have exercised this latter power to varying extents over the years.
Prior to passage of the Twenty-fifth Amendment, a vacancy in the office of the Vice President could not be filled until the next post-election inauguration. Such vacancies were common; sixteen occurred before the 25th Amendment was ratified–as a result of seven deaths, one resignation (John C. Calhoun, who resigned to become a U.S. Senator), and eight cases in which the vice president succeeded to the presidency. This amendment allowed for a vacancy to be filled with appointment by the President and confirmation by both chambers of the U.S. Congress. Since the Amendment's passage, two vice presidents have been appointed through this process, Gerald Ford in 1973 and Nelson Rockefeller in 1974.
The Twenty-fifth Amendment also established a procedure whereby a Vice President may, if the president is unable to discharge the powers and duties of the office, temporarily assume the powers and duties of the office as Acting President. George H. W. Bush did so once, on July 13, 1985. Dick Cheney did so twice, on June 29, 2002, and on July 21, 2007.