Water at high pressure and high temperature (hydrothermal conditions) is prevalent in geochemistry and may have contributed to the origin of life. On the practical side, hydrothermal methods are useful in selected applications for organic and inorganic synthesis, biomass conversion, and aqueous waste stream remediation. An overview is presented of fundamental research from this laboratory that is intended to underpin these fields. The objective is to determine the kinetics and outline the pathways of broadly applicable aqueous organic and inorganic reactions at the conditions of 373-623 K and 275 bar. The main approach employs IR spectroscopy in conjunction with a thin-film flow reactor for real-time, in situ observations of the species. Supplemental studies with Raman spectroscopy help identify the components. The molecular focus is on reactants that possess functional groups with universal implications for organic chemistry in water, e.g., amines, carboxylates, and nitriles.