Fatigue is a common presenting complaint in primary care offices. Nearly everyone experiences fatigue at one time or another, typically caused by overexertion or lack of sleep. Fatigue in the athlete is problematic as it may affect training and competitive performance. The possible causes of fatigue are widely varied, and the workup of fatigue can be very frustrating for the patient and physician alike. Making an accurate diagnosis is important to help the athlete return to effective training and maximal performance. Overtraining syndrome is a possible cause, but at this time remains a diagnosis of exclusion.