Homeothermy is defined by the Thermal Physiology Commission of the International Union of Physiological Sciences [35] as a pattern of temperature regulation in which cyclic variation in core temperature is maintained within arbitrary limits of ±2°C despite much larger variations in ambient temperature. At rest and in a neutral environment, deep body temperature may be kept within a much more narrow band of control (±0.3 °C). Claude Bernard’s concept of homeostasis depicting a virtual straight-line constancy is not, in fact, shown to be so precise, for there are spontaneous rhythmic variations in body temperature (the controlled variable) as the “set-point” of the thermoregulatory system undergoes changes with diurnal, monthly (e.g. ovulatory) or seasonal cycles. In the present chapter the principles of thermostasis are outlined with particular reference to the concept of set-point control in the central nervous system (CNS). Useful earlier general reviews of human temperature regulation may be found in Newburgh [55], Fox [20], Hensel [31] and Edholm and Weiner [15].