This chapter highlights the major consistencies and differences that are evident in the anatomy and physiology of those fish most likely to be encountered by the veterinarian or biologist working in the realm of aquatic animal health. It describes teleost fish, members of the infraclass Teleostei that includes bony fish with protrusible upper jaws, as these represent the majority of species commonly encountered in clinical or research settings. The chapter provides the more commonly encountered non-teleost fish including other bony fish like lungfish, sturgeons, and gars as well as the elasmobranchs. There are a number of external anatomical landmarks that are well conserved among fish. Fish may be subdivided into three anatomical regions: head, body, and tail. Sensory organs may figure prominently among the external anatomy of fish. A unique hallmark of the skin of many fish is their coloration, resultant from the pigment cells, or chromatophores, contained in the dermis.