Abiotic gaseous hydrocarbons comprise a fascinating, but poorly understood, group of Earth fluids generated by magmatic and gas–water–rock reactions that do not directly involve organic matter. At least nine different inorganic mechanisms, including Fischer-Tropsch type reactions, occur over a wide range of temperatures. Trace amounts (typically parts per million by volume) are formed in volcanic and geothermal fluids, but considerable amounts of methane, reaching 80–90 vol%, are now recognized in an increasing number of sites in Precambrian crystalline shields and serpentinized ultramafic rocks. Surface manifestations of abiotic gas related to serpentinization release gas directly to the atmosphere in ways that are similar to seepages of ordinary biotic gas from petroliferous areas. Abiotic methane is more widespread than previously thought. It also likely exists in sites undergoing active serpentinization and may be present in petroleum systems in the vicinity of serpentinized rocks.