High altitude illness - Acute Mountain Sickness (AMS), High Altitude Cerebral Edema (HACE) and High Altitude Pulmonary Edema (HAPE) - can be prevented or limited in severity by gradual ascent and by pharmacologic methods. The decision whether to use pharmacologic prophylaxis depends on the ascent rate and an individual's previous history of altitude illness. This review discusses risk stratification to determine whether to use pharmacologic prophylaxis and recommends specific drugs, especially acetazolamide, dexamethasone and nifedipine. This review also evaluates non-recommended drugs. In addition, this review suggests non-pharmacologic methods of decreasing the risk of severe altitude illness. There are also brief sections on how to decrease sleep disturbance at high altitude, travel to high altitude for patients with pre-existing illness and advice for travelers ascending to high altitude.