Before describing the potential of biomass to produce biocrude, chemistry around the entire process needs to be considered. This chapter presents an overview of the chemical considerations that must be taken into account to produce biocrude showing the crucial factors that influence its formation, composition and quality. Initially, a general description of the two main thermochemical processes (hydrothermal liquefaction and pyrolysis) to transform lignocellulosic and algae biomass is provided. The conversion of biomass into high-value-added products using both thermochemical processes is analyzed through the corresponding reaction mechanisms. These reaction mechanisms offer valuable insights into the chemical transformations occurring at each step, providing information to control and optimize the synthesis of desired products. Subsequently, an analysis of the chemical properties of different feedstocks and their relation with the biocrude quality is shown. Finally, concepts such as higher heating value, energy consumption ratio and reaction yield are also discussed considering an energy analysis. These analyses contribute to a better understanding of the chemistry behind biocrude production and provide a guide for the efficient design of sustainable products from biomass.