The Asilidae are the robber fly family, also called assassin flies. They are powerfully built, bristly flies with a short, stout proboscis enclosing the sharp, sucking hypopharynx. The name "robber flies" reflects their expert predatory habits; they feed mainly or exclusively on other insects and, as a rule, they wait in ambush and catch their prey in flight.

The Asilidae are a family in the order Diptera, the true flies. The common name for members of the family is robber flies, a name first suggested in 1869 by Alpheus Packard based on the German "Raubfliegen" (predatory flies). The Asilidae are cosmopolitan, with over 7000 described species. Latreille was the authority for establishing the family in 1802. The Asilidae, together with Bombyliidae and Therevidae, are the most representative families of the superfamily of Asiloidea and they form one of the most characteristic groups of the lower Brachycera.

Robber flies have stout, spiny legs and three simple eyes (ocelli) in a characteristic depression on the tops of their head between their two large compound eyes. They also have a dense moustache of stiff bristles on the face; this is called the mystax, a term derived from the Greek mystakos meaning "moustache" or "upper lip". The mystax has been suggested to afford some protection for the head and face when the flies deal with struggling prey; various Asilidae prey on formidable species including stinging Hymenoptera, powerful grasshoppers, dragonflies and even other Asilidae – practically anything of a suitable size. Some Asilidae do, however, specialize in smaller prey, and this is reflected in their more gracile build.

In general, the family attacks a very wide range of prey, including other flies, beetles, butterflies and moths, various bees, ants, dragonflies and damselflies, ichneumon wasps, grasshoppers, and some spiders. They seem to do so irrespective of any repugnatorial chemicals the prey may have at their disposal. When attacked, many Asilidae do not hesitate to defend themselves in turn using their probosces and may deliver intensely painful bites to humans if handled incautiously.

The antennae are short, have three segments, and sometimes bear a bristle-like structure called an arista.

Though they are a very characteristic group for such a large family, the Asilidae may easily be confused with the related and less widely known family Therevidae. Some points of contrast between the families include that the labium in the Therevidae is not a piercing, predatory organ, but ends in two fleshy labella adapted to the sucking of liquid foods. Again, the Therevidae commonly have fluffy setae above the mouthparts, unlike the stiff chaetae comprising the mystax of the Asilidae. Furthermore, in the Asilidae the depression on the vertex between the eyes tends to be more obvious than in the Therevidae.

The fly attacks its prey by stabbing it with its short, strong proboscis, injecting the victim with saliva containing neurotoxic and proteolytic enzymes which very rapidly paralyze the victim and soon digest the insides; the fly then sucks the liquefied material through the proboscis.

Many Asilidae have long, tapering abdomens, sometimes with a sword-like ovipositor. Others, for instance Laphria, are fat-bodied bumblebee mimics.