Hydrothermal gasification is the conversion of solid biomass into gaseous and/or liquid products in the presence of steam. Different hydrothermal biomass gasification processes are under development. In contrast to biomass gasification processes without water, biomass with the natural water content ("green biomass") can be converted completely and energeti­cally efficiently to gases. Depending on the reaction conditions, methane or hydrogen is the burnable gas produced. Some processes use catalysts. In recent years, significant progress was achieved in the development of various hydrothermal biomass gasification processes. However, some challenges still exist and technical solutions are needed before large-scale production facilities can be built (Kruse, 2009).