Aviation is an advanced technology, constantly pursuing higher performance, and backed by painstaking research into new materials and techniques. The target of “performance” has changed with the period, but the extent to which energy can be provided for propulsion has always been rated of major importance. Since propulsive energy is stored on board in a fuel medium, the more obvious criteria for the fuel are the energy density (based on mass and/or volume) and the proportion of this energy usable within the propulsion system. At the same time, the fuel must respond safely to reasonable handling procedures. The more subtle requirements to emerge over recent years include those of a minimum potential for pollutant formation and, in the case of very high speed flight, a maximum capacity for heat absorption.