Growth hormone (GH) is a polypeptide hormone produced by the somatotrope cells in the pituitary gland. It is the master anabolic hormone and possesses numerous bioactivities related to somatic growth, body composition, and intermediary metabolism. Many of the biologic actions of GH are mediated through insulin-like growth factor-I (IGF-I), but GH also has direct effects independent of IGF-I. Unlike most other hormones, GH is species specific, not only in its structure but also partially in its function. Its “one-way species specificity” refers to the fact that primate GHs are active in lower (evolutionarily earlier) species, but GHs of lower species are inactive in primates, including humans. GH regulation and, in part, GH action also differ among species. This chapter focuses primarily on human GH and its biology.