Until 1960's coaches and athletes believed that tolerate to thirsty and not to drink was an important factor to success in the endurance event. Some coaches even restricted their ath1etes to ingest fluid during the training. However, research data demonstrated that dehydration not only have negative impact on exercise performance but also on temperature regulation. This paper reviewed the effect of dehydration on exercise performance, specifically, in terms of aerobic exercise, physical work capacity, muscle strength, and anaerobic power. Research data and its implication to coaches and ath1etes are very important. The author suggest that athlete hydration status needs to be evaluated carefully, especially, using body weight as criteria to classify different competition events.