Travel to a high altitude requires that the human body acclimatize to hypobaric hypoxia. Failure to acclimatize results in three common but preventable maladies known collectively as high-altitude illness: acute mountain sickness (AMS), high-altitude cerebral edema (HACE), and high-altitude pulmonary edema (HAPE). Capillary leakage in the brain (AMS/HACE) or lungs (HAPE) accounts for these syndromes. The morbidity and mortality associated with high-altitude illness are significant and unfortunate, given they are preventable. Practitioners working in or advising those traveling to a high altitude must be familiar with the early recognition of symptoms, prompt and appropriate therapy, and proper preventative measures for high-altitude illness.