Methanogens are Archaea that produce methane as a metabolic by-product. They are common in wetlands, where they are responsible for marsh gas, and in the guts of animals such as ruminants and humans, where they are responsible for flatulence. Others are extremophiles, found in environments such as hot springs and undersea vents.

Methanogens are anaerobic. Some, called hydrotrophic, use carbon dioxide as a source of carbon and hydrogen as a reducing agent. Some of the carbon dioxide is used with the hydrogen to produce methane, which produces a proton motive force across a membrane, which is used to generate ATP. In contrast, plants and algae use water as their reducing agent. Other methanogens are acetotrophic, breaking down acetate to produce carbon dioxide and methane. A few methanogens are able to utilize methylated compunds such as methyalmines and methy thiols, as well.

There are over 50 species of methanogens, divided into three classes - the Methanobacteria , Methanococci , and Methanopyri. All are included among the Euryarchaeota. They are paraphyletic, and there is some reason to suppose the Methanococci are paraphyletic as well.