The family Cossidae contains over 110 genera with almost 700 known species. Carpenter millers are nocturnal and found worldwide. The exception is the Southeast Asian subfamily Ratardinae, which is mostly day flying. This family have large caterpillars and adults with a wingspan of 90–240 mm. These moths are mostly grey with some species having long, narrow wings resembling hawkmoths. Many are twig, bark, or leaf mimics. Cossidae often have some sort of large marking at the tip of the forewing uppersides which are conspicuous during flight, resembling a broken-off twig when the animal is resting.
Caterpillars are smooth with a few hairs and are usually tree borers. In some species, caterpillars take up to three years to mature. The caterpillars pupate within their tunnels and often have an unpleasant smell.