This chapter aims to present an overview of the hypotheses of fish phylogeny as a framework for understanding the remarkable diversity of fishes. The Chondrichthyes or cartilaginous fishes collectively are the extant sister group to the Osteichthyes. Lungfishes have true lungs that develop from an evagination of the pharynx, a partially divided atrium and ventricle in the heart, and a separation of pulmonary and systemic blood vessels with newly evolved pulmonary arteries. The Otocephala and its sistergroup the Euteleostei comprise all remaining fishes. Termed dragonfishes for the nightmarish teeth on their premaxilla and maxilla, stomiiforms are of particular interest because of bioluminescent organs and a mostly deep-sea lifestyle. The survey presented of the phylogeny and diversity of fishes is a small fraction of what is known about these fascinating aquatic animals. The bibliography included with J. S. Nelson et al. contains a wealth of resources for learning more, planning comparative studies, and accessing specialist literature.