Medical disorders related to environmental heat exposure are exceptionally common in persons who perform hard work in hot climates. They are also common in competitive athletes as well as in persons who participate in casual exercise to maintain health. The important issue of salt and water disturbances consequent to heavy sweating in hot climates is discussed in detail as are mechanisms of potassium deficiency and its implications. The major forms of environmental heat illness including heat syncope, heat cramp, heat exhaustion, and heat stroke are presented in detail with relevant clinical examples. A discussion of the differential diagnosis of hyperthermia and rhabdomyolysis follows. Because of the difference in treatment and complications, heat stroke is subdivided into the classic variety that affects the elderly and very young and that form that follows heavy physical work and is always associated with rhabdomyolysis. Because severe heat exhaustion and heat stroke are life-threatening disorders, the chapter includes a detailed discussion of complications and plans for treatment.