Catfish (order Siluriformes) are a diverse group of fish. Named for their prominent "barbels", which give the image of cat-like whiskers, they are found in freshwater environments of all kinds, with species on every continent except Antarctica. Some species from the families Ariidae and Plotosidae are also found in marine environments. They feature some of the smallest known vertebrates, including the candiru, the only vertebrate parasite to attack humans, as well as Pangasius gigas, the largest reported freshwater fish. There are armour plated types and also naked types, but they do not have scales. Not all catfish families have barbels; what defines a fish as being in the Siluriformes order are certain features of the skull and swimbladder.

Catfish have no scales. All catfish, except members of Malapteruridae (electric catfish), possess a strong, hollow, bonified leading ray on their dorsal and pectoral fins, through which a stinging protein can be delivered if the fish is irritated. In members of the family Plotosidae, and of the genus Heteropneustes, this protein is so strong it may hospitalize humans unfortunate enough to receive a sting.

Catfish range in size and behavior from the heaviest, the Giant Mekong Catfish (Pangasius gigas) in Southeast Asia and the longest Wels Catfish of Eurasia, to detritivores (species that eat dead material on the bottom), and even to a tiny parasitic species commonly called the candiru (Vandellia cirrhosa).

The wels catfish (Silurus glanis) is the only native catfish species in Europe, besides the much smaller related Aristotle catfish found in Greece. Mythology and literature record wels catfish of astounding proportions yet to be scientifically proved. The average size of the species is about 1,2m-1,6m, and fish more than 2m are very rare. The largest specimens on record measure more than 2.5 in length and sometimes exeeded 100kg. The wels catfish was introduced to Britain, Italy, Spain, Greece and some other countries during the last century. The species has flourished in the warm lakes and rivers of Southern Europe. The River Po in Italy and the River Ebro in Spain are famous for huge wels catfish, which grow up to 2m. These habitats contain plenty of food and lack natural predators.

Catfish, which have a sweet, mild flesh, are important food fish throughout the world. Ictalurids are cultivated in North America (especially in the Deep South), and representatives of the genus Ictalurus have been misguidedly introduced into European waters in the hope of obtaining a sporting and a food resource, however, the European stock of American catfishes have not achieved the dimensions of these fishes in their native waters, and have only increased the ecological pressure on native European fauna, while Clariids and Pangasiids are heavily cultured in Africa and Asia. There is also a large and growing ornamental fish trade, with hundreds of species of catfish, especially the genus Corydoras, being a popular component of many aquaria.

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