During the training season, a state of fatigue known as overtraining may occur, resulting from an excessive load of training, both in volume and intensity. Even now, difficult to predict the risk of overtraining, although this syndrome has been the subject of numerous studies. A lot of biological markers have been propounded. Taken alone, none of them have an absolute significance. This paper aims to review these markers, considering their biological interest, the ease with which they can be measured and the cost, from the simplest (body weight daily recording) to the most up to date markers (e.g. anti-oxidant status). They are grouped into three categories: non-invasive behavioural and biological markers, biochemical markers, and hormonal and immunological markers.