This advanced development program was built upon component technologies available at various levels of development at the time of its initiation. It was through modification of this essentially proven hardware that program risk and cost were minimized.The High Performance Auxiliary Power Unit (HPAPU) is based upon a modified Avco Lycoming 101 series gas turbine engine. The engine serves to drive a Sundstrand model 600D19CZ load compressor and model 60EG01 A-C generator through a Sundstrand gearbox adapted for the purpose. The resulting 500 hp class auxiliary power unit has successfully met or surpassed its required demonstration of duty cycles, continuous full power operation, and the ability to function over a wide environmental envelope. In addition, the HPAPU has exhibited a specific fuel consumption less than design and much less than the program's designated maximum allowable.This paper describes the HPAPU system. Characteristics which enhance its durability and maintainability are discussed. Performance data are presented which validate its capability from -70°F to +135°F and from sea level to 25,000 feet pressure altitude.