This article outlines the fundamental physiology and pathology of high altitude. It covers the latest findings from the unique experiment Operation Everest II in which eight persons were gradually decompressed to the "summit" of Mt. Everest and extensive physiologic studies done. The article contains an examination of the various high altitude pathologies, including acute mountain sickness, high altitude pulmonary edema, high altitude cerebral edema, and high altitude retinal hemorrhage. Information on prevention and current approaches to treatment conclude the article.