Supercritical fluids posses properties that make them attractive as media for chemical reactions. Conducting chemical reactions at supercritical conditions affords opportunities to manipulate the reaction environment (solvent properties) by manipulating pressure, to enhance the solubilities of reactants and products, to eliminate interphase transport limitations on reaction rates, and to integrate reaction and separation unit operations. Supercritical conditions may be advantageous for reactions involved in fuels processing, biomass conversion, biocatalysis, homogeneous and heterogeneous catalysis, environmental control, polymerization, materials synthesis, and chemical synthesis. Moreover, supercritical fluids can be used profitably in fundamental chemical investigations of intermolecular interactions and their influence on chemical processes. Work on chemical reactions in and with supercritical fluids is reviewed. We discuss both fundamental studies and applications of reactions at supercritical conditions, with focus on work published after 1985.