The President of the United States appoints the United States Secretary of Defense with the advice and consent of the U.S. Senate. The Senate provides its consent after conducting a confirmation hearing and holding a vote.
The Secretary of Defense is a member of the President's Cabinet. The Secretary also sits on the National Security Council.
In the United States, civilians control the military, and as a result, the Secretary of Defense must be a civilian. A retired member of the military may serve in this position, provided that seven years have passed since the individual was in active military service.