From small, general aviation aircraft to large commercial airlines, modern designs are increasingly being equipped with advanced systems that bolster performance and increase safety. Among the many systems that drive flight operations, the Auxiliary Power Unit (APU) is notable for its ability to support power and thrust needs without pure reliance on the main engines. While not every aircraft requires an APU, most commercial and many military platforms integrate them due to their valuable contributions to ground operations, inflight redundancy, and overall system reliability. As such, it can be beneficial for those involved in the industry to have an understanding of what an APU does, how it operates, and why manufacturers choose to include them in various designs.