Michael Mullen (b. 1946) is a retired United States Navy admiral who served as the 17th Chairman of the Joint Chiefs of Staff from October 1, 2007, to September 30, 2011, having previously served as the Navy's 28th Chief of Naval Operations. He was only the third officer in the Navy's history to be appointed to four different four-star assignments. Mullen retired from the Navy after over 43 years of service, later serving as a visiting professor at Princeton University.Photograph: United States Department of Defense