Have any former US Presidents served in the senate?

The short answer to the title of this article is: 16 presidents were senators at one time. They were: James Monroe, John Quincy Adams, Andrew Jackson, Martin Van Buren, William Henry Harrison, John Tyler, Franklin Pierce, James Buchanan, Andrew Johnson, Benjamin Harrison, Warren G. Harding, Harry S. Truman, John F. Kennedy, Lyndon B. Johnson , Richard M. Nixon and Barack Obama.