A mixture of methane (CH4) and carbon dioxide (CO2) produced by anaerobic digestion, with small amounts of other gases. The methane is a flammable gas, chemically identical to the main constituent of natural gas, and can be used as a fuel for heat and/or electricity generation. Biogas is effectively the same as landfill gas, which is produced by the anaerobic decomposition of organic material in landfill sites.