Hydrothermal liquefication (HTL) is a thermochemical process that occurs in the presence of water. HTL converts lignocellulose biomass (2nd generation biomass) into useful products via the breakage and polymerization of biomass at the cellular level into solid, liquid, and negligible gas products. The quality, quantity, and physical product state depend on reactor design and process parameters. HTL has the highest reported bio-oil yield (80 wt%) among thermochemical processes due to parameters optimization and water catalytic behavior. HTL’s wide range of applications sets it apart from other thermochemical processes. For instant, solid biochar and liquid products are used for water treatment, and fuel, respectively. However, products may need further treatment depending on the application. This review discusses thermochemical processes, feedstock categorization, HTL critical parameters, types of reactors, product applications, and limitations. Furthermore, this paper describes the mathematical and kinetic modeling steps to calculate process efficiency and reaction outcome.