The major physiologic stress encountered at high altitude is caused by the occurrence of hypobaric hypoxia. In this article, acute and chronic pulmonocardiac adaptation to altitude is reviewed, including possible genetic differences among highlanders from the Himalayan versus the Andean Mountains. The origin, symptoms, and treatment of acute mountain sickness and high altitude pulmonary edema are outlined. In addition, the prediction and prevention of pulmonary complications that may be encountered or exacerbated during commercial airflight are noticed.