This paper discusses the reasons for using aircraft APU’s and why a gas turbine power unit best meets the requirements for aircraft APU’s. This paper also reviews the history of APU’s in the military, commercial and business aircraft markets. A discussion of the various types of gas turbine APU’s is presented and a typical executive APU and secondary power system is described in some detail. Finally, a description of a new small APU, which is currently being certified for small turboprop and jet executive aircraft, is presented,