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 Vice President is the first person in the presidential line of succession, and assumes that office 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. U.S. Vice Presidents have exercised this latter power to varying extents over the years.
Prior to adoption of the Twenty-fifth Amendment in 1967, an intra-term vacancy in the office of the Vice President could not be filled until the next post-election inauguration. Several such vacancies occurred—eight vice presidents died, one resigned, and eight succeeded to the presidency. This amendment allowed for a vacancy to be filled through appointment by the President and confirmation by both chambers of the U.S. Congress. Since it's ratification, the vice presidency has been vacant twice (both in the context of scandals surrounding the Nixon Administration), and was filled both times through this process: in 1973, following Spiro Agnew's resignation, and again in 1974, after Gerald Ford succeeded to the presidency. The 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.
The persons who have served as vice president were born in or primarily affiliated with 27 states plus the District of Columbia. New York has produced the most of any state, as eight have been born there, and three others considered it their home state. Most U.S. Vice Presidents have been in their 50s or 60s and had political experience prior to assuming the office. The youngest person to become Vice President was John C. Breckinridge at 36 years of age, while the oldest was Alben W. Barkley at 71 years of age. Two U.S. Vice Presidents—George Clinton and John C. Calhoun—served under more than one President.