In this work, an overview of research and activity in the hydrothermal and supercritical fields is given. Properties and characteristics of water are reviewed in relation to separations, materials, and chemical conversion processes. Examples that are discussed include biomass fractionation, natural product extraction, material formation, biomass oxidation, biomass conversions, waste treatment, flames, and hydrocarbon upgrading. In the field of chromatography, water under hydrothermal conditions can be used to replace many organic solvents. As a solvent for separation processes, hot water can be used to achieve rapid and efficient fractionation of biomass or isolation of chemical compounds. In the field of materials, water in its supercritical state can be used to synthesize practical materials and to develop processes with low environmental burden. In the field of chemical processing, water can be used under oxidizing conditions to generate clean and renewable energy and to reduce the number of chemical steps for producing chemical products. A few other technological applications such as flames and hydrocarbon upgrading will be discussed. Water under hydrothermal, supercritical and high pressure conditions has many favorable properties that allow the development of green chemical processes and new technologies.