The potential of hot and supercritical water in applications to produce useful products, or to process unwanted compounds into environmentally compatible materials is reviewed. The potential of hot and supercritical water is high. Water changes its character from a solvent for ionic species at ambient conditions to a solvent for non-ionic species at supercritical conditions. Water at temperatures higher than ambient boiling temperature can be applied for extraction. At modest temperatures, ionic and polar species will be extracted. At higher temperatures, in particular approaching the critical temperature, nonpolar substances are readily dissolved and extracted. Hot pressurized water has a high reactivity. The reactions are commonly summarized as "hydrolysis reactions" which are catalyzed by acids, or may arise from simply hydrothermal transformations. Since CO2, dissolved in water increases the availability of protons, the addition of CO2 to liquid water catalyses hydrolysis reactions. Hydrolysis of natural plant materials provides a route to obtain fuel from non-food plant material. However, difficulties associated with operating conditions have so far limited the large scale implementations. (C) 2008 Elsevier B.V. All rights reserved.