Methanogen is a group of strictly anaerobic archaea, which can obtain energy for growth by formation of methane from simple compounds. The archaea is most likely linked to the origins and evolution of life on Earth, and plays an important role in global climate change, carbon biogeochemical cycle and resource utilization of agricultural wastes. The comprehensive understanding of biological characteristics of methanogens will promote the basic and applied research. This review summarizes recent studies of methanogens, including the physiological and chemical properties, metabolism pathway, energy conservation and systematic taxonomy.