The environmental impact of biomethanogenesis is related to its ecological role, accumulation and effect as a greenhouse gas, and application in anaerobic digestion for conversion of biomass and wastes to methane and compost. Biological formation of methane is the process by which bacteria decompose organic matter using carbon dioxide as an electron acceptor in the absence of dioxygen or other electron acceptors. This microbial activity is responsible for carbon recycling in anaerobic environments, including wetlands, rice fields, intestines of animals sediments, and manures. The mixed consortium of microorganisms involved includes a unique group of bacteria, the methanogens, which may be considered to be in a separate kingdom based on genetic and phylogenetic variance from all other life forms. Because methane is a significant and increasing greenhouse gas, its source fluxes and their potential reduction are of concern. Biomethanogenesis may be harnessed for reduction of wastes and conversion of renewable resources to significant quantities of substitute natural gas which could mitigate carbon dioxide and other pollutants related to use of fossil fuels.