The 22nd amendment limits the president to only two 4 year terms in office.  Before the 22nd amendment, Presidents traditionally served two terms, following the example of George Washington.  Franklin D Roosevelt broke this tradition during his presidency and served four terms, as World War II and the Great Depression convinced him to run for a third and fourth term, since the country was in crisis.  After FDR died in 1945, many Americans began to recognize that having a president serve more than eight years was bad for the country.  This led to the 22nd amendment, which was passed by Congress in 1947 and ratified by the states by 1951.