It is clear that high ambient or body temperatures can greatly impair athletic performance. Traditionally, the primary cause has been thought to be due to impaired cardiovascular delivery of blood to the muscles. However, many other factors are now known to contribute, from elevated brain temperature to altered neurochemistry to gut blood flow changes. This webinar will survey the multi-faceted impact of heat stress on human physiology. In the second part, major countermeasures for athletes will be discussed, including the efficacy of pre-cooling, the process of heat adaptation, and the role of hydration in optimal performance.