Heat stroke (HS) is a serious and potentially life-threatening condition defined as a core body temperature >40.6°C. Two forms of HS are recognized, classic heat stroke, usually occurring in very young or elderly persons, and exertional heat stroke, more common in physically active individuals. An elevated body temperature and neurologic dysfunction are necessary but not sufficient to diagnose HS. Associated clinical manifestations such as extreme fatigue; hot dry skin or heavy perspiration; nausea; vomiting; diarrhea; disorientation to person, place, or time; dizziness; uncoordinated movements; and reddened face are frequently observed. Potential complications related to severe HS are acute renal failure, disseminated intravascular coagulation, rhabdomyolysis, acute respiratory distress syndrome, acid-base disorders, and electrolyte disturbances. Long-term neurologic sequelae (varying degrees of irreversible brain injury) occur in approximately 20% of patients. The prognosis is optimal when HS is diagnosed early and management with cooling measures and fluid resuscitation and electrolyte replacement begins promptly. The prognosis is poorest when treatment is delayed >2 hours.