This text describes water's use in the production of raw fuels, as an energy carrier (e.g., hot water and steam), and as a reactant, reaction medium, and catalyst for the conversion of raw fuels to synthetic fuels. It explains how supercritical water is used to convert fossil- and bio-based feedstock to synthetic fuels in the presence and absence of a catalyst. It also explores water as a direct source of energy and fuel, such as hydrogen from water dissociation, methane from water-based clathrate molecules, and more.