Flight safety has been the primary driver for the development of aviation fuels and their governing specifications since first piston-engined aircraft and later turbine engine-aircraft were developed. For over 70 years in the case of turbine fuels, problems and issues with fuel storage, distribution and usage have led to the specifications being modified as much as the aircraft and engines have changed. Producing fuels to match current specifications reduces the risk of fuel-related problems not associated with handling challenges. This chapter provides an overview of fuel and specification changes, particularly in relation to civil jet fuels, and explains the main performance requirements embodied in the fuel specifications. These are living documents and nowadays allow for the use of synthetic fuel components; additional feedstocks and specification changes to accommodate them can be expected in response to environmental pressures.