This chapter discusses regulation of body temperature, where living tissues can function optimally only within a very narrow range of temperature. The temperature of the organs deep inside the body is quite close to the temperature of the blood, and is called the core temperature. The temperature of the superficial parts of the body, which is essentially the skin temperature, is called the shell temperature. In experimental studies, rectal temperature is a popular measurement. Regulation of body temperature depends on striking a balance between heat gain and heat loss. The effect of physiological and behavioural adaptive mechanisms eventually depends on how they affect heat transfer from or to the body by these physical processes. Information from all peripheral and central thermosensors is integrated in the central processors. Fever is usually associated with an inflammatory response in the body, which may or may not be due to an infection.