Core body temperature is one of the most tightly regulated parameters of human physiology. At any given time, body temperature differs from the expected value by no more than a few tenths of a degree. However, slight daily variations are due to circadian rhythm, and, in women, monthly variations are due to their menstrual cycle. Importantly, both anesthesia and surgery dramatically alter this delicate control, and as a result intraoperative core temperatures 1 to 3 degrees C below normal are not uncommon. Consequently, perioperative hypothermia leads to a number of complications including postoperative shivering (which unacceptably increases patients' metabolic rates), impaired coagulation, prolonged drug action, and negative postoperative nitrogen balance. In this review I will describe how anesthesia and surgery impair thermoregulation, the resulting changes in heat balance, and the physiological responses provoked by perioperative alterations in body temperature.