This article is about the insect. For the computer software, see GNAT.

The name gnat is applied to small insects in the order Diptera (the true Flies) and specifically within the suborder Nematocera . This suborder represents the more primitive members of the Dipteran order but still contains several very notable and important families such as the Chironomidae (non-biting midges) and the Culicidae (mosquitoes). Other families include the Tipulidae (crane flies), Bibionidae (hairflies), Ceratopogonidae (biting midges), Cecidomyidae (gall midges), Simuliidae (black flies), and others. The males often assemble together in large mating swarms. Gnat larvae are mostly free-living, many feeding on plants though some are carnivorous. Larval plant feeders, e.g., the Hessian fly larvae, cause root, stem, and leaf galls to be formed by the host plant. Some species of fungus gnats (families Mycetophilidae and Sciaridae) are very common pests of mushrooms and roots of potted plants in homes and greenhouses.

Some South American Pleurothallid orchids are pollinated by tiny gnats and have correspondingly small flowers.