Cardiac and kidney disease are common, increasingly encountered and often co-exist. Recently, the Acute Dialysis Quality Initiative (ADQI) Working Group convened a consensus conference to develop a classification scheme for the CRS and for five discrete subtypes. These CRS subtypes likely share pathophysiologic mechanisms, however, also have distinguishing clinical features, in terms of precipitating events, risk identification, natural history and outcomes. Knowledge of the epidemiology of heart–kidney interaction stratified by the proposed CRS subtypes is increasingly important for understanding the overall burden of disease for each CRS subtype, along with associated morbidity, mortality and health resource utilization. Likewise, an understanding of the epidemiology of CRS is necessary for characterizing whether there exists important knowledge gaps and to aid the in the design of clinical studies. In the most recent European and American guidelines for heart failure management, acute kidney injury and dysfunction were considered an index of poor prognosis. Paradoxically, however, in many randomized trials of interventions for patients with heart failure, those with kidney injury or dysfunction are often excluded. This review will provide a summary of the epidemiology of the cardio-renal syndrome and its subtypes.