Trends in increasing aircraft speeds and engine efficiencies are increasing vehicle and engine heat loads. Especially at higher Mach numbers, fuel is an attractive heat sink. For many vehicle applications, utilization of this heat sink would increase fuel temperatures beyond critical values, typically 370-400 degrees C (700-750 degrees F). As temperatures increase beyond about 480 degrees C (900 degrees F), this heat addition can lead to thermal/catalytic cracking of the fuel, leading to an "endothermic'' fuel. The principal barrier to the use of high temperature fuels is the deposition of carbonaceous material on heat exchanger passages, filters, fuel injectors, and other fuel system components. This paper will review progress in understanding and mitigating the thermal instability/deposition problem.