In this article, the importance of surfactant system in animals and human beings was reviewed. The surfactant is synthesized by pneumonocyte type-11 in the lung and consists rainly of lipids (90 %). This complex substance have an important role in reduction of surface tension, anti-edematous effect and facilitates air exchanges in alveoli. The composition of surfactant varies parallel with foetal development and it prepares the lung for postnatal respiration. Particularly in newborn child and animals, the surfactant deficiency causes important and fatal respiratory problems. On the other hand, many respiratory diseases can cause some changes in composition and quantity of surfactant in adults.