In aerospace applications, propulsion sets the most stringent requirement in terms of the levels of energy to be provided for that purpose, subject to the constraints of mass and volume available for carriage of the fuel within the vehicle. In this chapter, the following definitions are employed, with the maximum levels of net specific energy shown in parentheses: Conventional fuels—aviation fuel mixtures of a hydrocarbon nature invariably derived from petroleum (44 MJ/kg); Sustainable, alternative fuels—semi- or fully-synthetic fuels derived from alternative but sustainable feedstocks and meant to serve as blendstock or drop-in replacements for conventional fuels (44 MJ/kg); High-performance fuels—hydrogen, and individual hydrocarbon materials of particularly high energy content (120 MJ/kg); Substitute high-performance fuels—materials based on noncryogenic compounds of carbon, hydrogen, oxygen, nitrogen, and boron (68 MJ/kg). The performance of a bulk fuel in practice is a function of both the properties of the fuel in question and the conditions under which it is used. The former in turn depend on the nature and properties of the fuel components, whereas the latter influence the extent of intrinsic energy released and its conversion to produce vehicular thrust. This is particularly true with ramjet and rocket engine fuels because the chemical behavior of the combustion products within the duct between the combustion chamber and the thrust nozzle outlet can exert an overriding influence on the level of resultant thrust. This chapter provides a concise overview of the heat input requirements of high-performance engines together with the heat release available from candidate fuels. It also includes brief comment on the handling characteristics of these fuels to ensure that the most attractive candidates are not precluded from use by insurmountable problems within the distribution, storage, and vehicular fuel systems.