The overtraining syndrome is a chronic fatiguing illness that affects highly motivated endurance athletes. It is characterized by declining performance when maintaining a normal training program. The cause seems to be failure of adaptation within the neuroendocrine system, due to inadequate periods of rest. The onset is insidious and unfortunately, once it manifests, the only treatment is rest. This article discusses the known pathophysiology of overtraining, along with practical aspects of evaluation and treatment.