Book excerpt: The book provides fundamental chemistry and properties of near-critical water (NCW) and supercritical water (SCW), criteria and challenges/solutions in reactor design for NCW and SCW processes, and up-to-date reviews and practice of a wide range of their applications in bio refineries including: production of hydrochars from biomass, SCW oxidation (SCWO) for waste treatment, SCW gasification (SCWG) of biomass and waste for hydrogen and methane production, hydrothermal liquefaction of biomass, production of chemicals and SCWO of biofuels for energy. It also presents techno-economic analysis of hydrogen production via SCWG of biomass. The book will be highly essential for both academic researchers and industrial practitioners for developing novel bio refinery technologies and processes employing NCW or SCW for treatment of various organic waste streams and production of bio-energy and bio-based chemicals from bio-renewable resources. Prof. Dr. Zhen Fang is leader and founder of biomass group, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, China. Dr. Chunbao (Charles) Xu is currently an Associate Professor of Chemical Engineering and NSERC/FP Innovations Industrial Research Chair in Forest Bio refinery at Western University, Canada.