Mammals and birds are homeothermic—maintaining a constant internal body temperature. This homeostasis is disrupted, however, during surgery and anesthesia, when reduction in body temperature is common [1]. The fall in internal temperature may cause serious alterations in body metabolism and result in severe complications. The thermoregulatory system usually keeps central body temperature within about 0.2°C of “normal,” which is accepted to be about 37°C in humans. Temperature monitoring during general anesthesia is now routine. In this chapter, the physiology of body temperature and the effects of anesthesia on thermoregulation are discussed. Temperature monitoring methods are described, and various strategies for preventing and treating perioperative hypothermia in children are evaluated.