Hydrothermal processing has evolved as an alternative processing technology for wet biomass and waste materials in recent years. Using hot-compressed water as a reaction medium at temperatures of 200–500°C, materials with increased energy density can be obtained. The technology is particularly suited for wet and waste materials as drying of the feedstock is not required. Hydrothermal processing is divided into three separate areas depending on reaction severity: hydrothermal carbonization (HTC, 180–280°C), hydrothermal liquefaction (HTL, 280–375°C), and hydrothermal gasification (HTG, >350°C). Each of these hydrothermal routes results in energy densification by removal of oxygen to produce hydrochar (HTC), biocrude (HTL), or syngas (HTG). The process chemistry and reactions in hydrothermal media are described for each process. Suitable feedstocks and their considerations are reviewed as the quality of targeted biofuel is a function of feedstock and operating conditions. The quality of hydrochar influences its uses as a solid fuel while biocrude quality affects its use as a liquid fuel and feedstock for upgrading to drop-in replacement fuels, while HTG produces a syngas rich in either H2 or CH4. Hydrothermal processing results in a process water at all temperatures, typical decomposition products, treatments, and uses of the water byproduct are discussed. Advances in reactor design and scale-up efforts to demonstration and industrial scales are reviewed for each technology. An assessment is made of the current state of technology and further areas of research are discussed.