The effect of the composition of an aviation fuel, be it kerosene or gasoline, on the non-metallic componentsNon-metallic Components of the aircraft fuel system cannot be underestimated. Examples of unintended or undesirable effects of fuel compositional changes include leaking of fuel-wetted seals, swelling of polymeric fuel hoses, and removal of organic coatings from metal substrates. These events illuminate the importance of considering the impacts of changes in Fuel Compositionfuel formulation or composition. The complexity of the potential list of non-metallic materials, and determining the means of testing and evaluating data, is not immediately obvious. Considering the increasing contemporary efforts to develop alternative fuel sources, formulations, and compositions, the activities to support those challenges become more expansive, touching materials which may have a legacy of decades without being reviewed. While not an exhaustive discussion, the following chapter endeavors to provide a foundation for identifying challenges and developing the means for evaluating data.