Harold Brown was an American nuclear physicist who served as United States Secretary of Defense from 1977 to 1981, under President Jimmy Carter. Previously, in the John F. Kennedy and Lyndon B. Johnson administrations, he held the posts of Director of Defense Research and Engineering (1961–1965) and United States Secretary of the Air Force (1965–1969).

The United States secretary of defense (SecDef) is the head of the United States Department of Defense, the executive department of the U.S. Armed Forces, and is a high-ranking member of the federal cabinet. The secretary of defense's position of command and authority over the military is second only to that of the president of the United States, who is the commander-in-chief. This position corresponds to what is generally known as a defense minister in many other countries. The secretary of defense is appointed by the president with the advice and consent of the Senate, and is by custom a member of the Cabinet and by law a member of the National Security Council.