Cardiorenal syndrome is a spectrum of disorders that emphasizes the bidirectional nature of cardiac and kidney injury. Observational and retrospective studies have helped us to understand the prevalence and burden of each of the 5 types of cardiorenal syndromes. Cardiorenal syndrome type 1 is the most common. The nature of epidemiologic data limits clear delineation between cardiorenal syndrome types 2 and 4. Overall, the presence of cardiac or renal dysfunction strongly predicts a poor outcome of the contrary organ.