Apelin, the endogenous ligand for the G-protein-coupled APJ receptor, is emerging as a key hormone in cardiovascular homoeostasis. It is expressed in a diverse range of tissues with particular preponderance for the cardiovascular system, being found in both the heart and vasculature. Apelin is the most potent in vitro inotrope yet identified and causes endothelium- and nitric oxide-dependent vasodilatation. It also appears to have a role in lipid and glucose metabolism as well as fluid homoeostasis. One of the key emerging features of the apelin--APJ system is its interaction with the renin-angiotensin system with the respective receptors sharing marked sequence homology, forming heterodimers, and mediating opposing physiological actions. To date, both preclinical and limited clinical studies suggest that the apelin--APJ system may have an important role in the pathogenesis of heart failure. Although the apelin--APJ system is downregulated, the inotropic actions of apelin persist and are enhanced in failing hearts without inducing ventricular hypertrophy. In combination with its interaction with the renin-angiotensin system, APJ agonism may provide a new therapeutic target in the treatment of acute and chronic heart failure. In this review, we highlight key aspects of the apelin--APJ system in health and disease, and consider its translational and therapeutic potential. The diverse actions of the apelin--APJ system have implications for understanding the pathophysiology of, and development of treatments for, several major cardiovascular diseases.