The human body must maintain a relatively constant temperature to function. The thermoregulatory system plus human behavior control the balance of heat loss and heat gain. Heat is produced at a relatively constant rate by the body's basal metabolism. Heat production can be increased by increasing skeletal muscular activity. Heat is lost from the body to the environment by conduction, convection, radiation, and evaporation. Illness due to heat stress develops when the body gains more heat than it loses. Illness due to cold stress develops when the body loses more heat than it can produce or gain from external sources. During periods of extreme heat, mortality may rise sharply, sometimes to epidemic proportions. Because nearly all heat-related deaths are preventable, early detection of these deaths will allow appropriate preventive measures to be implemented. The autopsy findings in both heat-related and cold-related deaths are nonspecific. Information about environmental conditions, scene investigation, and the available medical history are very important in formulating the diagnosis.