Renal dysfunction is often present and/or worsens in patients with heart failure and this is associated with increased costs of care, complications and mortality. The cardiorenal syndrome can be defined as the presence or development of renal dysfunction in patients with heart failure. Its mechanisms are likely related to low cardiac output, increased venous congestion and renal venous pressure, neurohormonal and inflammatory activation and local changes, such as adenosine release. Many drugs, including loop diuretics, may contribute to worsening renal function through the activation of some of these mechanisms. Renal damage is conventionally defined by the increase in creatinine and blood urea nitrogen blood levels. However, these changes may be not related with renal injury or prognosis. New biomarkers of renal injury seem promising but still need to be validated. Thus, despite the epidemiological evidence, we are still lacking of satisfactory tools to assess renal injury and function and its prognostic significance.