A botfly is any fly in the family Oestridae, which includes all the members of the former families Cuterebridae, Gasterophilidae, and Hypodermatidae. It is the only family of flies whose larvae live as obligate parasites within the bodies of mammals, with the exception of a few screwworm flies in the family Calliphoridae. There are approximately 150 known species worldwide.

Dermatobia hominis, the human botfly, is the only species of botfly known to use humans as the host to its larvae.

Botflies deposit eggs on a host, or sometimes use an intermediate vector such as the common housefly, mosquitoes and even a species of tick (see Dermatobia hominis). The smaller fly is firmly held by the botfly female and rotated to a position where the botfly attaches some 30 eggs to the body under the wings. Larvae from these eggs, stimulated by the warmth and proximity of a large mammal host, drop onto its skin and burrow underneath. Intermediate vectors are often used since a number of animal hosts recognise the approach of a botfly and flee.

Eggs are deposited on animal skin directly, or the larvae hatch and drop from the eggs attached to the intermediate vector: the body heat of the host animal induces hatching upon contact or immediate proximity. Some forms of botfly also occur in the digestive tract after ingestion by licking.

The equine bot fly presents seasonal difficulties to equestrian caretakers, as it lays eggs on the insides of horse's front legs, on the cannon bone and knees, and sometimes on the throat or nose, depending on the species of bot fly. These eggs, which look like small, yellow drops of paint, must be carefully removed during the laying season (late summer and early fall) to prevent infestation in the horse. When a horse rubs its nose on its legs, the eggs are transferred to the mouth, and from there to the intestines, where the larvae grow and attach themselves to the stomach's lining or they pass into the small intestine and attach there. The attachment of the larvae to the tissue produces a mild irritation which results in erosions and ulcerations at this site. Removal of the eggs (which adhere to the host's hair) is difficult, since the bone and tendons are directly under the skin on the cannon bones: eggs must be removed with a sharp knife (often a razor blade) or rough sand paper, and caught before they reach the ground. The larvae remain attached and develop for 10–12 months before they are passed out in the feces. Occasionally horse owners will report seeing the bot fly larvae in horse manure. These larvae are cylindrical in shape and are reddish orange in color. In 1–2 months adult bot flies will emerge from the developing larvae and the cycle will repeat. Bots can be controlled with several types of dewormers, including dichlorvos, ivermectin and trichlorfon.

In cattle, the lesions caused by these flies can become infected by Mannheimia granulomatis, a bacterium that causes lechiguana, characterized by rapid growing, hard lumps beneath the skin of the animal. Without antibiotics an affected animal will die within 3–11 months.

The human botfly occasionally uses humans as the host to its larvae. The larva, because of its spines, can pose an extremely painful sub-epidermal condition. Removal processes include placing raw meat on to the area, which in theory will coax the larva out. Another option is to use the tree sap of the matatorsalo, found in Costa Rica, which is reputed to kill the larva, yet leave its body in the skin. Additionally, one can attempt to seal the breathing hole of the larva with nail polish or vaseline and then, after a day, squeeze out the suffocated, dead larva. Use of adhesive tape can work, but carries additional risk of infection because portions of the larva's breathing tube can be broken off by the tape and make the remainder of the body difficult to remove.

The sixth episode of season one of the television series "Beyond Survival" entitled "THE INUIT - SURVIVORS OF THE FUTURE" features survival expert Les Stroud and two Inuit guides hunting caribou on the northern coast of Baffin Island near Pond Inlet, Nunavut, Canada. Upon skinning and butchering of one of the animals numerous larvae (presumably Hypoderma tarandi although not explicitly stated) are apparent on the inside of the caribou pelt. Stroud and his two Inuit guides eat (albeit somewhat reluctantly) one larva each with Stroud commenting that the larva "tastes like milk" and was historically commonly consumed by the Inuit people.

Copious art dating back to the Pleistocene in Europe confirms their importance in premodern times as well.