The men and women who have served in the Senate are a varied group. As of 2005, the collective Senate membership includes two senators who began their Senate service at age 28 (though the Constitution mandates 30 as the minimum age), and one who was still serving at age 100. Fifteen senators later served as president, with one former president returning to serve in the Senate. Several senators represented two different states during their careers and one represented three. Before coming to Washington, three senators had participated in the Olympic Games. Four senators were awarded the Nobel Peace Prize.

These senators are among those included in the 2005 publication of the Biographical Directory of the United States Congress, 1774-2005. This edition, the culmination of more than seven years of work by the Senate Historical Office and the Office of History and Preservation of the House of Representatives, contains biographies of the nearly 12,000 individuals who served in the 1st through the 108th Congresses as well as the Continental Congresses between 1774 and 1789, and all vice presidents. The biographies are supplemented by a historical list of cabinet members and membership rosters of each Congress.

The origins of the Biographical Directory go back to 1859, when Charles Lanman, an author and former secretary to Daniel Webster, assembled the first collection of biographies of former and sitting Members for his Dictionary of Congress. With the creation of the Senate Historical Office in 1975 and the House Office of the Bicentennial in 1983, professional historians assumed responsibility for revising and updating the Directory. The results of their efforts appeared in the bicentennial edition of the Biographical Directory, which appeared in 1989. Since November 1998, the print edition of the Biographical Directory has been complemented and enhanced by an online database that is frequently updated.