This review covers published literature on the influence of whole-body hydration status on exercise performance. The majority of information in this area relates to endurance exercise performance, but information on power, strength, and sporting skills has also been investigated. These areas form the focus of the current review. It is apparent that some individuals can tolerate body water losses amounting to 2% of body mass without significant risk to physical well-being or endurance exercise performance when the environment is cold (for example 5 degrees C-10 degrees C) or temperate (for example 20 degrees C-22 degrees C). However, when exercising in a hot environment (an environmental temperature of 30 degrees C or more), dehydration by 2% of body mass impairs exercise performance and increases the possibility of suffering a heat injury.