Most of the energy is produced in the mitochondria in the cell by ‘burning’ glucose in oxygen and most is produced as heat. If the metabolic heat production was not released into the environment then a person at rest would increase in temperature and they would die of hyperthermia after about four hours. The ‘true’ metabolic heat production is the combined heat that the cells are producing and hence is related to the number of cells and how much each cell is producing. Different cells produce different amounts of heat and during activity, thermogenesis and shivering, muscle cells increase their heat production and dominate the contribution to metabolic rate. ISO 8996 provides a comprehensive set of standard values and methods, that can be useful for estimating the metabolic heat production for specific activities and populations. In addition to a detailed description of indirect calorimetry, methods proposed include estimates from a general description.