This book provides a detailed account of the research and applications of hydrothermal reaction technology in the utilization of biomass energy, encompassing processes such as bio-oil and hydrogen production. Hydrothermal conversion technology of biomass holds immense potential in the field of biomass energy conversion, including supercritical water gasification and subcritical hydrothermal liquefaction. The book covers recent advancements in biomass supercritical water gasification for hydrogen production, subcritical liquefaction for bio-oil generation, and bio-oil hydroprocessing, focusing on fundamental theoretical summaries and explanations. The book comprehensively and systematically summarizes the impacts of reaction conditions on the processes and product yield involved in biomass gasification, liquefaction, and bio-oil hydroprocessing. It delves into the reaction mechanisms, kinetic laws, catalysts employed in different reactions, and associated catalytic mechanisms. This book is beneficial for researchers and engineers in interdisciplinary fields such as new energy science, green chemistry and environmental engineering. It provides a systematic and comprehensive understanding of the fundamental concepts, principles, and latest developments in hydrothermal reactions. It enables researchers newly entering this field to swiftly initiate related studies and serves as a theoretical and technical reference for engineers and designers involved in the technology's engineering and process design.