Postnatal adaptation to air breathing necessitates the reduction of surface tension at the air-liquid interface in the alveoli of the lung. The rapid spreading and stability of phospholipids in pulmonary surfactant are mediated by the activity of two hydrophobic proteins, surfactant proteins B and C (SP-B and -C). Deficiency of surfactant proteins is associated with respiratory distress syndrome (RDS) in premature newborn infants, and in adults with adult respiratory distress syndrome (ARDS). While most surfactant deficiencies are secondary to prematurity or lung injury and infection, hereditary surfactant protein B deficiency, caused by mutations in the surfactant protein B gene, was recently identified as an inherited cause of respiratory failure in full-term newborn infants. This chapter describes the pathophysiology and molecular biology of hereditary SP-B deficiency.