Gas turbine auxiliary power units (APU) have been used in military aircraft for more than 20 years. This experience has demonstrated the reliability and utilitarian value of the gas turbine auxiliary power unit in providing a-c and d-c power, cabin pressurization, refrigeration, and pneumatic power for main engine starting. After many years of successful military use, the on-board gas turbine APU was utilized in several commercial airline applications. The inherent advantages of the gas turbine were successfully demonstrated on these initial applications. Today, all new jet transport air craft are utilizing the on-board APU concept of self-sufficiency. Only recently has this concept been initiated into the executive aircraft field. The advantages are even more desirable for this type of aircraft, since they frequently operate from stations without adequate ground support equipment. This paper discusses briefly the history of the on-board gas turbine APU, a variety of business and utility aircraft applications, and illustrates some of the typical installations. The operational and economic aspects of on-board gas turbines in this class of aircraft are also covered.