Pulmonary alveolar proteinosis (PAP) has been recognized for almost half a century. At least three separate pathophysiologic mechanisms may lead to the characteristic feature of PAP: the excessive accumulation of surfactant lipoprotein in pulmonary alveoli, with associated disturbance of pulmonary gas exchange. The prognosis for adult patients with PAP varies, but disease-specific survival rate exceeds 80% at 5 years. The survival rates for adult PAP patients seem to have increased progressively in the four decades since the initial clinical description of this condition. The last decade has brought new advances in laboratory and clinical research that are lifting a veil not only on PAP but also on general aspects of pulmonary surfactant biology and innate immune defense.