Thermoregulatory responses in infants and children are now fairly well understood. The phenomenon of heat loss in children during surgery is widely acknowledged. Hypothermia is most likely to occur during long surgical procedures in an air-conditioned operating room, particularly when respiration is controlled. Its consequences have prompted clinicians to take extra care in maintaining normothermia, especially in the very young. It is well known that anesthetized infants and children are capable of active thermoregulation and that their vasoconstrictive threshold differs little from that in adults [31]. Body temperatures can be measured using a variety of thermometers. Most clinically used thermometers are reasonably accurate. Perioperative hypothermia results from decreased metabolic heat production, increased environmental heat loss, redistribution of heat within the body, and anesthesia-induced inhibition of thermoregulation. Radiation and convection from the skin surface combine with evaporation from tissues inside surgical incisions to decrease mean body temperature. Perioperative hypothermia can be limited by prewarming the skin surface before induction of anesthesia, warming the operating room, humidifying the airway, and warming intravenous fluids.