Obesity has reached epidemic proportions in the United States and worldwide. Heart failure (HF) is also a major public health problem, which, despite therapeutic advances, is associated with substantial mortality. The adverse impact of obesity on the cardiovascular system is being increasingly recognized, and includes a hyperdynamic circulation, subclinical cardiac structural and functional changes, and overt HF. At the same time, the possible protective effect of obesity in patients with established HF has been emphasized in recent studies. This article reviews evidence from epidemiologic studies evaluating the impact of overweight and obesity on the risk of HF, appraises published data on the prognostic significance of overweight and obesity after the onset of HF, describes the potential mechanisms underlying these associations,speculates on the clinical implications of current evidence, and suggests directions for future research.