[en] Optimal training results in exercise-recovery harmonious balance. High frequency and volumic training, associated with insufficient recovery may induce overtraining. It depends on the kind of exercise but occurs mainly in endurance sports. The high frequency of upper respiratory track infection in endurance sportsmen results from a temporary immunosuppression state, possibly associated with overtraining. Many immune and haematological parameters have been associated with overtraining, but none seemed really efficient for diagnosis. Hormonal parameters, despite contradictions in studies, appear to give a good insight of athlete's staleness and are more predictive of overtraining. Actually, overtraining detection needs a combination of different parameters, and cannot sustain on isolated factors.