Franklin D. Roosevelt is the only person ever to have been elected to more than two terms as U.S. President, serving from March 1933 until April 1945.
Up until then, the two-term tradition had been an unwritten rule. Ulysses S. Grant and Theodore Roosevelt had both tried to run for a third term, and had been widely criticised for it. Facing challenges from prominent Democrats, Roosevelt moved the party's convention to Chicago, where he had a big following. He announced that he would not run unless it was what the delegates wanted. The delegates then ecstatically nominated him. Roosevelt won the general election, but died during his third term. He was succeeded by his Vice-President, Harry S. Truman.
In 1947, the U.S. Congress passed the Twenty-second Amendment to the U.S. Constitution, limiting Presidents to two terms in office. In the event of a Vice-President succeeding the office, two years is considered a 'term'. However, this clause was specifically waived in the case of President Truman, allowing the Amendment's passage.