Hyperthermia is defined as a body temperature greater than 40 degrees C. Several conditions can cause hyperthermia. In sepsis, the immunologic reaction to the infection most often manifests as a fever. Some toxic ingestions and withdrawal states can cause elevated body temperature. Certain medications can cause a hyperthermic response, such as in neuroleptic malignant syndrome. The most common disease that can be treated by cooling alone is heat-related illness and heat stroke. Heat-related illness is a spectrum of disease that occurs when the body's thermoregulatory system does not work properly. Heat exhaustion is characterized by elevated core body temperature associated with orthostatic hypotension, tachycardia, diaphoresis, and tachypnea. Heat stroke is defined as elevated core body temperature plus central nervous system involvement (delirium, decreased the level of consciousness, or ataxia). Heat-related illness most often affects athletes (exertional hyperthermia), but can also occur during the warm weather months or in locations with extreme temperatures. Patients with impaired thermoregulation (those at extremes of age, the obese or mentally ill) are at higher risk. The definitive treatment for heat-related illness is total body cooling. Conduction and evaporation are the two modes of cooling employed in the treatment of heat-related illness. Studies have shown ice water immersion to be the most effective and most rapid. However, there are obvious barriers to performing this in an emergency department. Marathons and other athletic events that have frequent heat-related illness sometimes have this capability. Evaporation (mist and fan) is the second most rapid way to cool a patient. Ice packs to the groin, axilla, neck, and areas near other great vessels have been shown to be less effective. Cooled intravenous fluids have been studied, but there is no clear consensus regarding their benefit (preservation of neurologic function) versus potential harm (induced shivering), but they may be considered. This article will discuss the procedure for performing evaporative cooling with other adjuncts in the field. The priority in heat-related illness is early recognition and intervention. Military and sports literature has identified 40 degrees C as the target, and the faster the target is achieved, the lower the patient mortality.