GaN based solid state power amplifiers (SSPAs) have extremely attractive properties, making them ideal for use in both ground applications and space-based satellite communication systems. However, there is a limited amount of information available on the reliability of this new technology. The purpose of this paper is to describe the underlying complexities associated with GaN based SSPAs, and to present the methodology and findings of an extensive R&D program that correlated theoretical results with field data, demonstrating the reliability of GaN based SSPAs.