Proteinuria is one of the most common manifestations of renal disease. Changes in plasma protein concentration, glomerular permeability, tubular reabsorption, or renal hemodynamics can affect the rate of urinary excretion and the composition of urinary proteins. Determination of the significance of the proteinuria requires an evaluation of the amount and type of proteinuria in the context of the clinical setting. In addition, the clinician must be familiar with the laboratory evaluation of proteinuria and its limitations in order to correctly interpret these tests.