A large consumption of H-2 affects the overall economy of conventional hydroprocessing. The costs can be decreased by using water as the source of active hydrogen. This can be achieved under subcritical and supercritical water conditions providing that an active and stable catalyst is developed. Hydroprocessing in aqueous phase has been studied for potential applications in upgrading of high oxygen content feeds and heavy petroleum feeds to liquid hydrocarbons. The feeds were tested at temperatures ranging from less than 200 to 500 degrees C and total pressure from 1 to 30 MPa. These conditions cover subcritical and supercritical regions of water. Water takes part in hydroprocessing reactions as a free radical scavenger and a hydrogen donor. Hydrogen generated in situ via partial reforming and water-gas shift reactions is more reactive than external hydrogen. Catalyst development for hydroprocessing in aqueous phase has been receiving much attention. High performance was observed over the catalysts containing noble metals (Pt, Pd, Ru, and Rh) supported on various supports; however, the information on a long-term stability of these catalysts is limited.