Molecular evidence suggests that in spite of the great morphological diversity in the order, all catfish form a monophyletic group.

The earliest known definitive members lived in the Americas from the Campanian to Maastrichtian stages of the Late Cretaceous, including the Andinichthyidae, Vorhisia vulpes and possibly Arius.

[1][12][13] A potential fossil record is known from the earlier Coniacian-Santonian stages in Niger of West Africa,[2] though this has been considered unreliable,[13] and the putative earliest armored catfish known from the fossil record, Afrocascudo, lived during the Cenomanian age of the Late Cretaceous in Morocco of North Africa (Kem Kem Group).

[3] The describers of Afrocascudo claimed that the presence of a derived loricariid so early on would indicate the extensive diversification of catfish, or at least loricarioids, prior to the beginning of the Late Cretaceous.

[3] Britz and colleagues suggested that Afrocascudo instead represents a juvenile obaichthyid lepisosteiform, possibly a junior synonym of Obaichthys.

FishBase and the Integrated Taxonomic Information System lists Parakysidae as a separate family, while this group is included under Akysidae by both Nelson (2006) and ACSI.

[19] The family Horabagridae, including Horabagrus, Pseudeutropius, and Platytropius, is not shown by some authors but presented by others as a true group.

[25][26][27] Molecular evidence usually contrasts with this hypothesis, and shows the suborder Loricarioidei as the earliest branching catfish lineage, and sister to a clade that includes the Diplomystidae and Siluroidei; this phylogeny has been obtained in numerous studies based on genetic data.

[24] When a data filtering method[29] was used to reduce lineage rate heterogeneity (the potential source of bias) on their dataset, a final phylogeny was recovered which showed the Diplomystidae are the earliest-branching catfish, followed by Loricarioidei and Siluroidei as sister lineages, providing both morphological and molecular support for Diplomystidae being the earliest branching catfish.

[32] †Andinichthyidae Nematogenyidae Trichomycteridae Callichthyidae Astroblepidae Loricariidae Diplomystidae †Bachmanniidae †Hypsidoridae Cetopsidae Siluridae Pangasiidae Mochokidae Claroteidae Plotosidae Ictaluridae Clariidae Ailiidae Sisoridae Bagridae Aspredinidae Doradidae Auchenipteridae Heptapteridae Pseudopimelodidae Pimelodidae Unassigned families: Extant catfish species live inland or in coastal waters of every continent except Antarctica.

[37][38] In the Southern United States, catfish species may be known by a variety of slang names, such as "mud cat", "polliwogs", or "chuckleheads".

In general, they are negatively buoyant, which means that they usually sink rather than float due to a reduced gas bladder and a heavy, bony head.

Some have a mouth that can expand to a large size and contains no incisiform teeth; catfish generally feed through suction or gulping rather than biting and cutting prey.

[33] In some catfish, the skin is covered in bony plates called scutes; some form of body armor appears in various ways within the order.

[50] Juvenile catfish, like most fish, have relatively large heads, eyes, and posterior median fins in comparison to larger, more mature individuals.

These juveniles can be readily placed in their families, particularly those with highly derived fin or body shapes; in some cases, identification of the genus is possible.

Exceptions to this are the ariid catfish, where the young retain yolk sacs late into juvenile stages, and many pimelodids, which may have elongated barbels and fin filaments or coloration patterns.

[54] In North America, the largest Ictalurus furcatus (blue catfish) caught in the Missouri River on 20 July 2010, weighed 59 kg (130 lb).

These records pale in comparison to a Mekong giant catfish caught in northern Thailand on 1 May 2005, and reported to the press almost 2 months later, that weighed 293 kilograms (646 lb).

[55] Also in Asia, Jeremy Wade caught a 75.5-kilogram (166.4 lb) goonch following three fatal attacks on humans in the Kali River on the India-Nepal border.

[58] Spermatocysts are formed from cytoplasmic extensions of Sertoli cells; the release of spermatozoa is allowed by breaking of the cyst walls.

[58] The occurrence of seminal vesicles, in spite of their interspecific variability in size, gross morphology, and function, has not been related to the mode of fertilization.

Many species in the catfish family possess an enhanced first pectoral fin ray, called the spine, which can be moved by large abductor and adductor muscles.

In some catfish, pectoral fins are longer in males than in females of similar length, and differences in the characteristic of the sounds produced were also observed.

In several different species of catfish, aggressive sound production occurs during cover site defense or during threats from other fish.

[66] Catfish raised in inland tanks or channels are usually considered safe for the environment, since their waste and disease should be contained and not spread to the wild.

Catfish have widely been caught and farmed for food for thousands of years in Africa, Asia, Europe, and North America.

[70] In Indonesia, catfish is usually served fried or grilled in street stalls called warung and eaten with vegetables, sambal (a spicy relish or sauce), and usually nasi uduk (traditional coconut rice).

In Bangladesh and the Indian states of Odisha, West Bengal and Assam, catfish (locally known as magur) is eaten as a favored delicacy during the monsoons.

In Myanmar (formerly Burma), catfish is usually used in mohinga, a traditional noodle fish soup cooked with lemon grass, ginger, garlic, pepper, banana stem, onions, and other local ingredients.