The high prevalence of obesity and diabetes will lead to higher rates of morbidity and mortality. The search for drugs to treat these metabolic disorders has, therefore, intensified. The stomach-derived peptide ghrelin regulates food intake and body weight. Recent work suggests that ghrelin also controls glucose metabolism. In addition, current evidence suggests that most of the actions of ghrelin could contribute to the metabolic syndrome. The ghrelin signaling system is, therefore, a promising target for the development of new drugs for the treatment of obesity and diabetes. Agents that block the ghrelin signaling system might be especially useful targets. This Review summarizes the potential and the limitations of ghrelin as a tool to better understand, prevent and treat obesity and diabetes.