This chapter explains hydrothermal processing of heavy oil and bitumen, with emphasis on supercritical water (SCW) and subcritical water as solvent, reactors, catalysts, and in hydrogen production. Petroleum are naturally occurring hydrocarbon materials containing variable amounts of oxygen, nitrogen, sulfur, and other elements. Bitumen, also called native asphalt, is a collective term for all naturally occurring reddish-brown to black materials of semisolid, viscous to brittle character that can contain up to 50 wt% mineral. The use of SCW may provide a way to upgrade heavy oil. The role of SCW and subcritical water on heavy oil upgrading is still unclear despite their use as solvent. SCW and subcritical water form a homogeneous phase with heavy oil. Hydrogen is a crucial component in heavy oils upgrading in SCW, driving both economics and environmental effects. Enough supply of hydrogen from SCW and subcritical water ensures effective conversion of heavy oils to light oil fractions, coke suppression, and heteroatoms removal.