Anaerobic digestion (AD) is a biochemical process that harnesses the complex metabolic reactions of a specific community of synergistic microorganisms. Whilst AD is an effective process for deriving energy from purpose-grown biomass, this process also plays an important role in recovering the energy from organic wastes and capturing and treating the greenhouse gases (GHGs) associated with their disposal in landfill. This chapter outlines the key biochemical reactions involved in the anaerobic digestion of biomass, typical designs and applications and how they are monitored, controlled and optimised. There are four key phases of anaerobic digestion, namely hydrolysis, acidogenesis, acetogenesis and methanogenesis. Anaerobic digestion is perhaps one of the most common applications for the treatment of sewage sludge as it reduces final sludge volumes, limits odours and destroys pathogens. The chapter shows an overview of different anaerobic treatment technologies.