Organic waste may degrade anaerobically in nature as well as in engineered systems. The latter is called anaerobic digestion or biogasification. Anaerobic digestion produces two main outputs: An energy-rich gas called biogas and an effluent. The effluent, which may be a solid as well as liquid with very little dry matter may also be called a digest. The digest should not be termed compost unless it specifically has been composted in an aerated step. This chapter describes the basic processes of anaerobic digestion. Chapter 9.5 describes the anaerobic treatment technologies, and Chapter 9.6 addresses the mass balances and environmental aspects of anaerobic digestion. This chapter contains sections titled: Definition of Anaerobic Digestion, Biogas and Digestate The Structure of Anaerobic Digestion Estimation of Methane Potential and Gas Composition Process Factors References