Cystatin C (cysC) is an endogenous cysteine peptidase (proteinase) inhibitor that has been intensively investigated as a biomarker of kidney function for many years. It is freely filtered at the kidney glomerulus because of its small size and lack of cysC binding protein. A very small amount of cysC appears in the urine because of its total reabsorption in the proximal tubule and lack of secretion. Since 1985 cystatin C has been studied mainly in serum/plasma as an alternative biomarker to serum creatinine to estimate GFR. In this review we present the recent discoveries concerning urinary cystatin C determination as a tubular injury biomarker.