Methanogens are prokaryotic microorganisms that produce methane as an end-product of a complex biochemical pathway. They are strictly anaerobic archaea and occupy a wide variety of anoxic environments. Methanogens also thrive in the cytoplasm of anaerobic unicellular eukaryotes and in the gastrointestinal tracts of animals and humans. The symbiotic methanogens in the gastrointestinal tracts of ruminants and other “methanogenic” mammals contribute significantly to the global methane budget; especially therumen hosts an impressive diversity of methanogens. This makes this updated volume an interesting read for scientists and students in Microbiology and Physiology.