The interactions between the heart and the kidney recently have been the focus of intense interest because of epidemiological evidence indicating that even mild deterioration of renal function is an important risk factor for poor outcome in patients with congestive heart failure, myocardial infarction, and cardiovascular surgery. Kidney function deterioration may be a consequence of cardiac and baroreceptor dysfunction or may be primarily caused by intrinsic kidney disease. This review provides a comprehensive analysis of the role of the kidney not only as a marker but also as a pathogenic factor in cardiorenal syndromes, whether primary heart or primary kidney disease or both are the initiators of the subsequent pathophysiological events.