The term “jet fuel stability” covers a variety of fuel properties that result from the fuel’s chemical composition upon production, as well as changes to these properties upon fuel degradation in the supply chain and upon use in aircraft engines. Stability is ordinarily separated into storage stability, the ability of a fuel to resist degradation during supply chain transport and storage, and thermal stability, the ability of the fuel to resist degradation during exposure to relatively high temperatures. This chapter summarizes the fuel degradation processes which occur in these two regimes, details the test methods employed, and offers guidance in preventing and mitigating the effects of these degradation mechanisms.