Microbial methanogenesis accounts for approximately 74% of natural methane emission. The process plays a major role in global warming and is important for bioenergy production. This paper reviews the biochemical pathways of methanogenesis. It is currently recognized that methanogenesis proceeds via three biochemical pathways depending on the carbon sources, including hydrogenotrophic, aceticlastic, and methylotrophic methanogenesis. Multiple enzymes and coenzymes are involved in the process, during which Na~+ or proton gradient is created across the cell membrane, contributing to limited ATP synthe sis. In the hydrogenotrophic pathway, CO_2 is reduced to methane with H_2 or formate as an electron donor. In the aceticlastic pathway, acetate is split into methyl and carboxyl group, then the carboxyl group is oxidized to produce H2 which is used as the electron donor to reduce methyl group. In the methylotrophic pathway, methyl group is reduced with external H2 or reducing equivalent from the oxidation of its own methyl group. The ATP gained from per mol substrate for different pathways are as follows: hydrogenotrophic > methylotrophic > aceticlastic pathway. Due to the unculturability of most archaeal methanogens, understandings of the biochemical pathways of methanogenesis and the relationships between methanogens and other microbial communities will have to depend on new technologies including bioinformatics, gene engineering and metabolic modelling.