Voluntary heat production is brought about by exercise or physical activity, whereas involuntary heat production results from shivering or the secretion of hormones, such as thyroxine and catecholamines. This chapter illustrates the potential avenues for heat exchange in an exercising human and these heat exchange avenues include radiation, conduction, convection, and evaporation. Evaporation of sweat from the skin is dependent on three factors: the temperature and relative humidity; the convective current around the body, and the amount of skin surface exposed to the environment. The dehydration poses further problems for the athlete because progressive dehydration impairs the ability to sweat and, consequently, to thermoregulate. The dehydration-induced impairment in exercise performance is much more likely in hot environments than in cool conditions, which implies that altered thermoregulation is an important factor responsible for the reduced exercise performance associated with dehydration.