Pulmonary surfactant, a phospholipid-protein complex that lines the alveoli and small airways of the lungs, is secreted by type ll alveolar epithelial cells. Recognized functions of surfactant are (1) maintaining proper humidity of the alveolar wall; (2) providing a constant essential factor for easier pulmonary elastic recoil and reducing the work of breathing during passive expiration; (3) stabilizing the alveoli by lowering the surface tension at the air-liquid interface, or decreasing the tendency of small alveoli to collapse and fuse into larger alveoli at expiration; (4) resisting infiltration of capillary and interstitial fluids into the alveoli; and (5) emulsifying very small inhaled particles that may have reached the alveoli and thus facilitating their phagocytosis by macrophages. lt is stated that deficiencies of surfactant or differences in its composition are concerned with morphofunctional immaturity of lung in prenatal or neonatal period and many pulmonary diseases of adults. Therefore, many investigation are performed on the roles and therapeutic features of surfactant in these diseases and new information is added to old ones in every day.