A gene encoding a novel class a G-protein-coupled receptor was discovered in 1993 by homology cloning and was called APJ. It was designated an "orphan" receptor until 1998, when its endogenous ligand was identified and named apelin (for APJ endogenous ligand). Since this pairing, both apelin and its receptor have been found to have a widespread distribution in both the central nervous system and the periphery. A number of physiological and pathophysiological roles for the receptor have emerged, including regulation of cardiovascular function, fluid homeostasis, and the adipoinsular axis. This review outlines the official International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification nomenclature, designating the receptor protein as the apelin receptor, together with current knowledge of its pharmacology, distribution, and functions.