Among the world's largest flies are the horseflies (family Tabanidae). Though not all the species in this family bite, these large, hairy flies are most often known as pests because of the painful bites many species can inflict on animals and humans. They occur worldwide, being absent only at extreme northern and southern latitudes.

A type of insect, horseflies are classified in the fly order Diptera. There are approximately 3,000 species of horseflies known worldwide, 350 of which are found in North America. At least three subfamilies are recognised:

• Chrysopsinae
• Pangoniinae
• Tabaninae
• the genus Zophina is of uncertain placement, though it has been classified among the Pangoniinae.

The two best-known types are the common horseflies, genus Tabanus, and the deerflies, genus Chrysops, also known as banded horseflies because of their coloring. Both these genera give their names to subfamilies.

Adult horseflies feed on nectar and other plant juices, but only the females also feed on blood. Males lack the necessary mouth apparatus to do so. Most horseflies feed on mammal blood, but some species are known to feed on birds, amphibians or reptiles.

The females' primary sense for locating prey is sight, and they have large, compound eyes that serve this purpose well. The flies usually lay waiting in shady areas for prey to happen by. They are attracted to large, dark objects, and to certain animal odors and carbon dioxide. They are also attracted by motion, their eyes being well adapted to its detection.

The eyes of horseflies are generally brightly colored, and this coloration is the primary means entomologists use to sex them.

A horsefly's bite can be very painful. Unlike insects that pierce the skin, horseflies have mouthparts that work like miniature knives, which they use to slash open the skin with a scissor-like motion. This causes the blood to seep out as the horsefly licks it up. While some horseflies are known to have venom, none is known to be dangerous to humans. When attacking humans, the flies generally prefer the head and upper body regions, going unnoticed until a bite is inflicted.

Horseflies are most active in hot weather, mostly in summer and autumn during the daylight hours. Most species also prefer a wet climate, which makes it easier for them to breed.

Eggs are generally laid on stones close to water or on plant stems or leaves. On hatching, the larva fall into water or moist earth, feeding voraciously on invertebrates, such as snails and earthworms, and small vertebrates.

Some horsefly species are known to transmit disease and/or parasites. A common problem in some animals, though, when large flies are abundant, is blood loss. Some animals have been known to lose up to 300mL of blood in a single day, which can severely weaken or even kill them.