Ghrelin, an endogenous ligand for the growth hormone secretagogue receptor, is synthesized principally in the stomach and is released in response to fasting. Ghrelin is structurally related to motilin and, together, they represent a novel family of gut–brain regulatory peptides. In addition to having a powerful effect on the secretion of growth hormone, ghrelin stimulates energy production and signals directly to the hypothalamic regulatory nuclei that control energy homeostasis. The study of ghrelin has extended our understanding of how growth is controlled, and has shown that the stomach is an important component of this system.