The Cecidomyiinae, commonly known as gall midges or gall gnats, is the largest subfamily in Cecidomyiidae with over 600 genera and more than 5000 described species. Larvae of the other (basal) cecidomyiid subfamilies feed on fungi; whereas this subfamily is best known for its members that induce galls on plants. However, there are also many species of Cecidomyiinae that are fungivores, parasitoids, or predators as maggots.

Herbivorous species make up about 75% of the Cecidomyiinae; many are gall-inducers, infecting a wide diversity of plants. They are found world-wide and are the most ecologically diverse gall-inducing group of organisms. Most develop in one or a few closely related host-plant species. In three tribes (Asphondyliini, Alycaulini and Lasiopterini) the midges are associated with specific fungi; these have been termed 'ambrosia gallers' because the fungi line the inner walls of the galls, in a similar way to those in the galleries of ambrosia beetles. Some of the herbivorous species are inquilines, developing in galls that were initiated by a different midge species. Most species in the genus Macrolabis and the tribes Trotteriini and Camptoneuromyiini are inquilines, but there are also many inquiline species in genera that are predominantly gall-inducing (such as Dasineura and Lasioptera).

Predatory and parasitoid species comprise about 12% of the Cecidomyiinae. Larvae of these species attack other arthropods - there are known predators of mites, aphids, scale insects, dragonfly eggs, and endoparasitoids of Hemiptera. Most are found in the tribes Aphidoletini and Lestodiplosini.

Fungivores make up much of the remainder of the subfamily, indicating the retention of (or reversion to) the ancestral food source.

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