Methanogenesis is the process where methane is formed by microbial methanogens. Methanogenesis is a form of anaerobic respiration when oxygen is not used. Many methanogens are known to be obligatory anaerobes and are inhibited in growth in the presence of oxygen. In this process, the terminal electron acceptor is carbon such as acetic acid and carbon dioxide. Other carbon electron acceptors include formic acid, methanol, methylamine, dimethyl sulfide, and methanethiol. The methyl compounds are catabolized by methyl transferases.

Methanogenesis is important in that it is the final step in the degradation of biomass and organic matter. When the biomass decays, inorganic electron acceptors including sulfur, oxygen, iron, and nitrate are depleted and H2 and CO2 accumulates. Fermentation and methanogenesis takes over as the prominent catabolic processes at this stage. Methanogens are able to remove hydrogen, and carbon dioxide to flush out excess carbon accumulation in anaerobic environments.