A biomass system converts organic matter, such as crop residue or manure, into a form that can readily be used for electricity generation. In the context of off-grid systems, biomass is converted to biogas, syngas, or dried and processed for direct combustion. This chapter introduces the fundamental processes involved in the production of biogas and syngas using anaerobic digestion and gasification. The analysis and design of biogas reactors is presented. Economic, environmental, and social considerations of the use of biomass for off-grid electrification are discussed.