An Auxiliary Power Unit (APU) is a relatively small self-contained generator used in aircraft to start the main engines, usually with compressed air, and to provide electrical power while the aircraft is on the ground. In many aircraft, the APU can also provide electrical power in the air.

An APU was first used on the Pemberton-Billing P.B.31 Night Hawk Scout aircraft in 1916.

Although APUs have been installed in many locations on various military and commercial aircraft, they are usually mounted at the rear of modern jet airliners. The APU exhaust can be seen on most modern airliners as a small pipe exiting at the aircraft tail.

In most cases the APU is powered by a small gas turbine engine that provides compressed air from within or drives an air compressor (load compressor). Recent designs have started to explore the use of the Wankel engine in this role. The Wankel offers power-to-weight ratios better than normal piston engines and better fuel economy than a turbine.

APUs fitted to ETOPS airplanes are more critical than others as they supply backup electrical and compressed air in place of the dead engine during emergency. While most APUs may or may not be flight startable, such APUs must be flight startable at all altitudes. If such APUs malfunction, the airplane cannot be released for ETOPS flight and is forced to take a longer route.

APUs are even more critical for space shuttle flight operations, since the main engines are designed only for liftoff. The APUs provide power for most vehicle subsystems, including landing gear and brakes, rocket engine gimballing, and moving the shuttle's control surfaces.