During prolonged exercise or work in the heat, human thermal homeostasis is first challenged, and eventually lost, as one moves from a compensable state through to uncompensable heat stress. During the first week of such exposure, work and athletic performance is most affected, and the threat of heat illness is greatest. However, given adequate time, the body will undergo a three-phase adaptation to better tolerate the heat. In this review, the principles and practices of the six primary methods by which such heat adaptation may be achieved are evaluated. One technique involves repeated exposure to both heat and exercise, and is designed to elevate and maintain a target body temperature, by varying the intensity of the work rate during the acclimation period: the controlled-hyperthermia (isothermal) technique. It is recommended that this method provides the most dependable, and least hazardous, means of adapting workers and athletes for heat stress.