The jet engine group comprises aero turbines, ramjets and rockets, their level of performance increasing in that order, with fuel requirements showing both similarities and differences. The conventional fuel for aero turbine engines, for example, is aviation kerosine, several variants of which exist for specific applications. Aviation fuel specifications are invariably stringent, and variations with density are shown for typical properties. The dwindling availability of optimal crudes over the last 25 years has resulted in a general degradation in the quality of aviation kerosine, with adverse effects on combustion performance, emissions and engine life except where hardware solutions emerged in parallel. In fact, the reduction of emissions is seen to be more a matter of engine design than fuel technology. In the near term, supplies of kerosine may be supplemented from sources other than crude oil, whereas in the longer term, kerosine may be substituted by liquid methane and/or liquid hydrogen. In comparison with kerosine, liquid hydrogen produces more nitrogen in its combustion products on a fuel mass basis, but less on an energy basis, although the flame temperature is higher giving possibilities of more NOx. The fuel requirements of high energy content and storage stability apply across the board, but additional parameters of concern are heat capacity in the case of ramjets, and combustion-product chemistry with rockets, which demand a range of candidate high-performance fuels.