SUMMARY. Pulmonary surfactant is a complex mixture of lipids and proteins that constitutes the mobile liquid phase covering the large surface area of the alveolar epithelium. It maintains minimal surface tension within the lungs in order to avoid lung collapse during respiration. The components of surfactant are synthesised within the lung in the alveolar type II cells, Clara cells and submucosal cells. Surfactant components are also produced in small quantities at non-pulmonary sites (e.g. trachea, brain, testes, salivary glands, lachrymal glands, heart, prostate, kidney, pancreas). Pulmonary surfactant is composed of phospholipids, neutral lipids and surfactant proteins (SP). The SPs are divided into the hydrophobic SP-B and SP-C and the hydrophilic SP-A and SP-D. SP-B and SP-C lower the alveolar surface tension, while SP-A and SP-D are primarily concerned with host-defense functions, acting as immune mediators. They may aggregate or agglutinate pathogens, recruit and activate neutrophils and macrophages, induce phagocytosis, offer resistance to allergen challenge by interfering with allergen–IgE interaction, mast cell/basophil degranulation, cellular infiltration and helper Tcell polarization, and they are involved in the clearance of apoptotic and necrotic cells. Pulmonary surfactant abnormalities have been implicated in various human diseases, such as obstructive lung disease (asthma, bronchiolitis, chronic obstructive pulmonary disease [COPD]), infectious and suppurative lung diseases (cystic fibrosis [CF], pneumonia, and human immune deficiency virus [HIV]), adult respiratory distress syndrome (ARDS), pulmonary oedema, chronic lung disease of prematurity and SP-B deficiency, interstitial lung diseases (sarcoidosis, idiopathic pulmonary fibrosis [IPF], hypersensitivity pneumonitis), pulmonary alveolar proteinosis, and following cardiopulmonary bypass and lung transplantation, and in smokers. Pneumon 2007; 20(4):364–371