A Fleet Replacement Squadron (FRS), is a unit of the United States Navy and Marine Corps that trains Naval Aviators and Naval Flight Officers (NFOs) on the specific front-line aircraft they have been assigned to fly. Students, referred to as Replacement Pilots or Replacement Weapon Systems Officers are either newly winged aviators (Category I), aviators transitioning from one type aircraft to another (Category II), or aviators returning to the cockpit after a period of non-flying (Category III). After completing the training regimen, graduates are assigned to fleet squadrons. Additionally, FRSs are responsible for training aircraft mechanics, providing replacement aircraft for fleet squadron attrition, and standardizing maintenance and aircraft operations.
U.S. Navy and Marine personnel (RPs, instructors, maintainers, etc.) are frequently assigned to other services FRSs. The FRSs were formerly known as Replacement Air Groups, and are thus commonly called "Rags".