Mammals and birds are homeothermic, requiring a nearly constant internal body temperature. When internal body temperature deviates significantly from normal, metabolic functions usually deteriorate, and death may result. The thermoregulatory system usually maintains core body temperature within 0.2C of “normal” which is about 37C in humans. Alteration of normal body temperature is a major underestimated, and often undertreated threat to the anesthetized patient(1). Anesthetic induced inhibition of thermoregulation combines with exposure to a cold operating room enviroment to make most unwarmed patient hypothermic. In recent years, major outcome studies have shown that mild hypothermia ( 1-2C) (1) triples the incidence of morbid cardiac outcomes, (2) triples the incidence of surgical wound infections and prolongs hospitalization 20 percent, and (3) significantly increases surgical blood loss and the need for blood transfusions. Hypothermia is by far the most common perianesthetic thermal perturbations. However, hyperthermia is more dangerous than, to a comparable degree of, hypothermia. An understanding of normal and anesthetic-influenced thermoregultion facilitates prevention and management of these and numerous other temperature-related complications.