Four environmental conditions contribute to climatic heat stress: ambient temperature, air humidity, air movement, and radiant heat from the sun and nearby warm surfaces. If sufficiently great, this climatic heat stress can adversely affect the performance of work or exercise. Particularly for endurance sport activities, such as marathon running, racewalking, and other sustained distance events, the ability to quantify the magnitude of heat stress is of value in helping both coach and athlete plan the safest competitive strategy. Quantification of climatic heat stress is possible through the use of three thermometers, which measure shade temperature (dry bulb temperature [DBT]), the effect of wind and humidity (wet bulb temperature [WBT]), and radiant energy (black globe temperature [BGT]). The three values are incorporated into a heat stress index (HSI), a temperature value comprised of 10% of DBT, 20% of BGT, and 70% of WBT. Extensive HSI monitoring was performed at the 1992 Barcelona Olympic Games, on‐site during all of the endurance competitions (10, 20, 50 km racewalks, marathons, and modem pentathlon cross country). These data provided the first in‐depth assessment of the extent to which the athlete is able to compete successfully under conditions ranging from high risk to extreme risk heat stress with minimal actual heat injury. This success is due in part to the adaptation in tolerating heat stress that occurs with training, as well as the instinctive reduction in pace adopted by the athlete to permit the optimum balance of skin and muscle blood flow that keeps core temperature from rising too high.