This chapter focuses on one of the actinopterygian groups, the teleosts, which is the largest group of vertebrates with more than 25,000 species. The term “fish” covers multiple taxonomic groups of aquatic vertebrates with a variety of phylogenetic positions, such as cyclostomes, chondrichthyans, actinopterygians, and sarcopterygians. Neuroendocrine axes are responsible for the control of major physiological functions and life traits in vertebrates, such as growth, metabolism, reproduction, stress, metamorphosis, and migration. Growth hormone is perhaps the most pleiotropic pituitary hormone, as in vertebrates it is involved in the control not only of growth but also of reproduction, metabolism, and immunity, as well as osmoregulation in teleosts. Compared with tetrapods, much less is known concerning the brain component of the thyrotropic axis in teleosts. Typical larval metamorphosis, also referred to as “primary metamorphosis”, occurs in the life cycle of some teleost species, such as elopomorphs and pleuronectiforms.