Under the physiological conditions, protein excretion in urine is less than 150 mg per day. Proteinuria is defined as an excessive rate of protein excretion in the urine. Measurement of proteinuria and identification of its components are important for the diagnosis of various clinical settings. Proteinuria may be the first finding for severe renal damage and its follow-up is necessary for monitoring the treatment. In this review, the classical and current data about the physiological protein excretion, proteinuria types, proteinuria mechanisms explained in several diseases, the relationship between proteinuria and renal damage and finally methods for urinary protein determination were discussed.