The order of Catfishes (Siluriformes) is one of the largest in number of families, genera, and species. The group is found in most freshwaters and shallow saltwater bodies around the world. Its extraordinary evolutionary and biogeographic pathways leave many open questions on its origin and large diversification that make its study a complex subject, yet a fascinating field for research. Catfishes, a Highly Diversified Group is divided into two volumes. Volume 1 (Their Outstanding Biology, 14 chapters) aims to rectify the incomplete information on the enormous morphological diversity of the group, special habits and adaptations to specific environments, and morpho-functional characteristics of the catfishes. Volume 2 (Evolution and Phylogeny, 18 chapters) deals with evolutionary mechanisms and major evolutionary trends within Siluriformes, investigating new avenues concerning fossil and extant catfishes and relationships within and among families, using morphological and molecular evidence.