The flying qualities design of the Northrop YF-17 lightweight fighter was based on in-flight and ground-based simulation data. In this paper some of the more significant design aspects of the flying qualities are discussed. The need for highly responsive uncoupled maneuvering capability in fighter aircraft is identified. The solution used in the YF-17 is presented. A brief description of the airplane and its flight control system is followed by a discussion of the generation of the flying qualities criteria in the preliminary design phase of the airplane's development. The key part played by ground-based and in-flight simulation data is identified. This is followed by a discussion on the use of the Northrop Large Amplitude Flight Simulator in the refinement of the flying qualities criteria and the development of the flight control system, to satisfy the criteria. Subsequent successful flight test of two YF-17 prototypes validated the use of the simulator in the design process.