The catfish belong to the super-order Ostariophysi, order Siluriformes, and are grouped under 31 families. The principal families covered are as follows:
- Callichthyidae – Small armoured catfish.
- Siluridae – Catfish with very long anal fins and small, or absent, dorsal fins.
- Doradidae – Heavy bodied with a single row of bony plates, every of which bear a spine, running along the centre of every side of the fish.
- Bagridae – Unarmoured catfish with long barbels. Old World.
- Pimelodidae – Unarmoured catfish with long barbels. New World.
- Bunocephalidae – Unarmoured catfish with flat heads.
- I.oricariidae – Spiny armoured catfish with sucking disc mouths.
Catfish never have scales, but some species are partly or totally covered with bony plates, and also the vast majority have barbels.