Heat is produced in the cells of the living body and much of the investigation into human response to thermal environments can be regarded as the study of the distribution and dispersion of that heat. It is often considered that heat is a by-product from cells; however, this is misleading, especially in homeotherms where the heat production is an important integral part of the living system. If no heat were produced, as in death, the body temperature would change to achieve thermal equilibrium with surrounding conditions. Metabolic heat that is produced by the living body is used to maintain an internal body temperature of around 37°C and the body will attempt to preserve or lose sufficient heat to the environment to try to maintain that temperature. An estimate of metabolic heat production in the body is therefore fundamental to the assessment of human thermal environments.