A gnat is a small flying insect that belongs to the dipterid suborder Nematocera. They can be both biting and non-biting, and often fly in large numbers, called clouds. Gnats are a loose descriptive category rather than a phylogenetic or other technical term, so there is no scientific consensus on what constitutes a gnat. Some entomologists consider only non-biting flies to be gnats. Certain universities and institutes also distinguish eye gnats, which are non-biting flies attracted to fluids secreted by the eyes.

Gnats have antennae with at least six segments that are often long and slender, and they are generally slender-bodied with long and narrow wings. Black fly (Simuliidae) and biting midges (Ceratopogonidae), also belonging to the gnat category, are small, sometimes barely visible, blood-sucking flies commonly known in many areas as biting gnats, sand flies, punkies, or "no-see-ums," among other names.

Gnats can be beneficial or detrimental depending on the species. Some species are excellent plant pollinators and feed on crop pests such as aphids and scales, while others, such as the Hessian fly, are crop pests themselves. Depending on the species, gnats can be biting or non-biting and will feed on plants, other insects, or blood.

Gnats are found anyplace in the world where there is a river or stream because they lay their eggs in watery places. Some species of gnats need the blood of warm-blooded animals to survive. Unlike mosquitoes, gnats usually dont bite through clothing, but they can crawl into hair or under clothing to get at places such as ankles and belt lines.

To control adult gnats in smaller areas, pressurized aerosol sprays with pyrethrins can be used. Other control measures in the household can include turning off unnecessary lights at dusk and sealing cracks and crevices where gnats can enter. Most of these methods address only individual, adult specimens. In order to properly treat an entire gnat infestation, including larvae, it is best to consult a pest control professional.