Bat flies are highly modified true flies. They are members of the order Diptera, and currently are grouped into two families, the Nycteribiidae, and the Streblidae. They are also bloodsucking ectoparasites, living in the fur, or on the wing membranes of bats.

Nycteribiidae are wingless, have reduced eyes, and a spiderlike appearance. This is because their legs actually insert dorsally into the thorax.

Streblidae have varying degrees of wing reduction, and appear more like a regular fly. They also have reduced eyes.

All bat flies are viviparous, meaning that a third larval stage is borne from the female fly, which is then glued to a structure in the surroundings of the roost. Once deposited, the larva immediately pupates. Depending on the species, these pupae may be located close to the roost, or at quite some distance from the roost.

Preliminary phylogenetic studies (Dittmar et al., 2006), show that Streblidae is not monophyletic, and rather, the bat flies follow an Old World/ New World division. This makes for interesting evolutionary scenarios, especially considering the evolution of their hosts - the bats.