Biogas, a mixture of methane and carbon dioxide, is produced by anaerobic digestion of organic waste. Municipal and agroindustrial organic wastes are important feedstocks for producing biogas. Anaerobic digestion is a microbial process. It not only reduces the amount of waste to be disposed, but the methane produced can be burnt as fuel to provide energy. Anaerobic digestion is carried out in various kinds of bioreactors. This chapter is focused on the diverse bioreactor systems, or digesters, that are available for producing biogas. Depending on their design and operation, the different types of bioreactors are suited to different kinds of wastes, processing scenarios and other conditions. The following main types of bioreactors are discussed: the conventional anaerobic digesters (e.g. anaerobic sequencing batch reactor, continuous stirred tank reactor, anaerobic plug-flow reactor); the sludge retention reactors (e.g. anaerobic contact reactor, up-flow anaerobic sludge blanket reactor, expanded granular sludge blanket reactor, up-flow anaerobic solid-state reactor, anaerobic baffled reactor, internal circulation reactor, anaerobic fluidized bed reactor); the anaerobic membrane reactors; the anaerobic biofilm reactors; and the high rate reactors.