This handbook covers the British species of the subfamily Mycetophilinae of the family Mycetophilidae, commonly known as the fungus gnats. Most fungus gnats are found in forests and other wooded habitats, as might be expected since the larval development of many species is dependent on saproxylic or mycorrhizal fungi. Fungus gnats may be found throughout the year, their relative abundance affected by prevailing weather conditions and the consequent effect on the availability of fungus hosts.