Organohalogens like perchloroethene and trichloroethene are prominent groundwater pollutants due to their industrial use as dry cleaning and degreasing agents and their widespread release into the environment. Volatile organohalogens like chloromethane strongly influence atmospheric chemistry and thereby Earth's climate by causing ozone depletion when released into the atmosphere. For a long time it was assumed that these compounds are only produced and released by human activity. However, in recent years, over 5,000 naturally-occurring organohalogen compounds have been identified, and evidence suggests that the cycling of halogens e.g. chlorine, bromine in soils is largely driven by microbial processes.