An overview is given of our present knowledge on taxonomy, phylogeny and biogeography of catfishes or Siluroidei, one of the worlds economically important groups of fresh and brackish water fishes. In an introductory chapter, the taxonomic position of catfishes in the ichthyological classification is discussed; their external morphology is briefly described and synapomorphies showing their monophyly are listed. In the following chapter, cattish systematics are presented. Thirty three families with 416 genera and 2 584 species are presently recognized, but these numbers are continuously changing; it is therefore obvious that the systematics of many taxa are still rather poorly known. For each family, data (if any) supporting their monophyly are given together with a short description of their external morphology. The valid genera are listed, with for each genus, the number of presently recognized species with a reference (if any) to a recent revisionary study. Contributions to the phylogenetic study of catfishes are scarce, and dealing only with a few families and with a limited number of taxa within the families. As a results only few data on catfish phylogeny are presently known. Therefore, also our knowledge on historical biogeography of catfishes is seriously limited, until more date become available on interfamilial and intergeneric relationships.