The overtraining syndrome affects mainly endurance athletes. It is a condition of chronic fatigue, underperformance, and an increased vulnerability to infection leading to recurrent infections. It is not yet known exactly how the stress of hard training and competition leads to the observed spectrum of symptoms. Psychological, endocrinogical, physiological, and immunological factors all play a role in the failure to recover from exercise. Careful monitoring of athletes and their response to training may help to prevent the overtraining syndrome. With a very careful exercise regimen and regeneration strategies, symptoms normally resolve in 6-12 weeks but may continue much longer or recur if athletes return to hard training too soon.