This article provides an overview of the field of catalysis in sub- and supercritical water. Contributions that are particularly important, either for historical or scientific reasons, are highlighted. Areas where additional research is needed are identified. Work in this field is categorized into three major areas: chemical synthesis, waste treatment, and biomass processing. Given the opportunities that catalysis in supercritical water provides for advances in green chemistry/engineering and sustainable energy, one can expect even more research and development activity in this growing field. (C) 2008 Elsevier B.V. All rights reserved.