MX designations were a series of designations used for experimental weapon system programs, including jet- and rocket-powered precision-guided munitions, designed and manufactured by U.S. defense contractors under contracts provided by the United States Army Air Force (1945–1947), the United States Air Force (1947–1954) and the United States Navy. Some of those missile projects, which were equally applicable for naval uses (e.g. air-to-surface anti-ship missiles and air-to-underwater anti-submarine missiles,) eventually have been transferred to the Navy, and later developed under the auspices of Naval research facilities.
The "MX" designation was a temporary one that was eventually recognised as a standard designation for certain military research and development. The "MX" indexation system was in use by the USAF Air Materiel Command from the very beginning of the United States rocket programs until July 1, 1954, but certain weapon programs have been indexed "MX" during their project development since 1954, such as the SM-65 Atlas (MX-1593). The LGM-118 Peacekeeper was given the project title MX but was in no way part of the MX series of designations.