The cardiorenal syndrome is a clinical manifestation of the bidirectional interaction between the heart and kidneys. Evaluating renal function is an essential part of the assessment of every cardiac patient. It has become clear that serum creatinine is not an accurate enough marker of glomerular filtration rate (GFR) and should not be used to evaluate kidney dysfunction. Creatinine-based estimates of GFR are preferred, but require renal function to be stable and are not suitable when changes in kidney function occur. Cystatin C (CysC) has been the target of much interest in the search for an alternative measure of GFR. As an endogenous biomarker, CysC possesses many of the properties required of a good marker of renal function. Compared with that of creatinine, plasma concentrations of CysC are less influenced by factors other than GFR. Consequently, CysC correlates with true GFR more accurately than creatinine. Equations for estimating GFR from CysC values have also been developed, which makes values easier to interpret and facilitates the clinical use of this new marker. The use of CysC in acute kidney injury has also shown promising results. CysC has been studied as a risk marker for prognosis in cardiovascular disease. This effect is attributed to the strong impact of renal dysfunction on progressive cardiovascular disease and impaired survival. Higher levels of CysC have consistently been predictive of incident or recurrent cardiovascular events and adverse outcomes. CysC is a predictor of the development of heart failure and increased levels of CysC have an independent association with higher mortality in both chronic and acute heart failure. In conclusion, CysC appears to be an interesting marker of renal function and is useful for risk stratification in heart failure.