The Northrop T-38 Talon is an American supersonic jet trainer. It was the world's first supersonic trainer and is also the most produced to date. It remains in service as of 2010 in air forces throughout the world. The United States Air Force (USAF) is the largest user. In addition to USAF pilots, the T-38 is used in the United States by NASA astronauts, with the aircraft bailed to NASA from USAF. The U.S. Naval Test Pilot School is the principal U.S. Navy operator (other T-38s were previously used as USN aggressor aircraft until replaced by the similar F-5 Tiger II), as well as some NATO pilots participating in joint training programs, also fly the T-38.