Hydrothermal gasification is treatment of biomass in hot compressed water, usually above 350oC and above 20 MPa to obtain combustible gas. Figure 4.5.1 shows phase diagram of water, where gas-liquid equilibrium line starts from the triple point and ends at the critical point. Hydrothermal condition is located around the critical point. When both temperature and pressure is higher than critical temperature and critical pressure, respectively, the state is called supercritical water, and gasification in supercritical water is called “supercritical water gasification”. This hot compressed water enjoys high reactivity, and when biomass is placed in this water, it is gasified by hydrolysis and pyrolysis reactions.