 The gastrointestinal peptide hormone ghrelin was discovered in 1999 as the endogenous ligand of the growth hormone secretogog receptor. Increasing evidence supports more complicated and nuanced roles for the hormone, which go beyond the regulation of systemic energy metabolism. The hormone has diverse biological functions, including learning and memory and gut motility. This article was authored by T. D. Muller, R. Nogares, M. L. Anderman, and others.