Biomass is a useful feed material for energy and chemical resources. Hydrothermal gasification of biomass wastes has been identified as a possible system for producing hydrogen. Supercritical and subcritical water has attracted much attention as an environmentally benign reaction medium and reactant. The main objective of this study is to assess and introduce the hydrothermal gasification of biomass wastes containing various quantities of the model compounds and real biomass. The decomposition of biomass, as a basis of hydrothermal treatment of organic wastes, is introduced. To eliminate chars and tars formation and obtain higher yields of hydrogen, catalyzed hydrothermal gasification of biomass wastes is summarized.