Cystatin C is a low molecular weight-protein, which may replace creatinine for the evaluation of renal function, particularly in the clinical settings where the relationship between creatinine production and muscular mass impairs the clinical performance of creatinine. This paper intends to summarize the current knowledge about the physiology of cystatin C and about its use as a renal marker, alone or within formulas developed to estimate the glomerular filtration rate. Moreover, this paper reviews the recent data about potential other applications of cystatin C, especially in cardiology, in oncology and in clinical pharmacology.