Aviation Additivesfuel specifications set out minimum requirements for a given liquid to be accepted as fit for purpose for use in aircraft. These requirements are a combination of compositional restraints, physical properties and performance criteria that, historically, have proven to produce a fuel that is acceptable for Aviation. Additives have been incorporated into the fuel specifications to help address inherent functional and performance limitations with the fuel. Aviation fuel additives, whether deliberately added or present because of crossover from other fuels during storage and transportation, are tightly controlled in jet fuel specifications. The chapter describes additive usage as well as the approval processes involved for aviation additives to be accepted into aviation fuels, focussing on jet fuel applications. Challenges with additive usage are also addressed and ways to understand, prevent or mitigate the issues are discussed.