The Model GTCP331 Auxiliary Power Unit (APU) is designed to meet the stringent demands of the next generation of commercial jet transports. The APU provides compressed air for cabin air conditioning, main engine starting, standby hydraulic power (via a turbopump) and in-flight anti-icing, as well as electrical power for both ground and flight operation. The GTCP331 will provide low cost of ownership, fuel efficiency and environmentally pleasing operation in the new Airbus A.300/A.310 and Boeing 767/757 aircraft.This paper presents information related to various aspects of the GTCP331 APU program. Specifically, data related to the configuration and power class selection, propulsion engine heritage design details, and development highlights are presented. The interrelationship of the APU to other aircraft systems and the role of the APU digital Controller in assuring efficient and safe operation are discussed.