An athlete who trains intensely, yet consistently underperforms, is considered to be suffering from overtraining syndrome (OTS). OTS is a complex state that involves a large variety of signs and symptoms. Symptoms include changes in mood or behaviour, decreases or increases in concentration of different blood molecules, and alterations in immune function. Although several hypotheses have been proposed, each only explains a selective aspect of OTS. Presently, the sole agreement is that OTS is associated with excessive training and insufficient rest and recovery. The hypothesis proposed in this paper suggests that excessive training/competing causes repetitive tissue trauma, either to muscle and/or connective tissue and/or to bony structures, and that this results in chronic inflammation. It is further proposed that traumatized tissue synthesizes a group of inflammatory molecules, cytokines. Cytokines have been shown to coordinate the different systems of the body to promote recovery. Suggestions are made to detect, prevent, and rehabilitate the overtrained athlete.