Multiple exostosis disease is one of the hereditary diseases with autosomal dominant transmission. It is characterized by the proliferation of bone protuberances, especially located in the metaphysis of long bones.Since 1993, advances have been noted in knowledge of the pathophysiology of this disease, in particular with the discovery of the mutation of EXT genes, found in 80% of multiple exostosis disease. These genes, tumor suppressors, code for proteins involved in the synthesis of heparan sulfates. The deficiency in quantity and quality of heparan sulfates leads to changes in certain metabolic processes, which leads to the development of ectopic growth plaques. This is responsible for the development of exostosis, but also for the low longitudinal growth of long bones. The disease phenotype may also associate abnormalities in the shape and length of long bones, such as the typical "Bessel Hagen" deformity. Clinically, bone masses are often painless. The rare complication (2 to 5% of cases) but the most feared is the transformation into chondrosarcoma, which motivates regular clinical and radiological monitoring of these patients. Treatment, mainly surgical, is indicated in case of symptoms (pain, increased exostosis volume after the end of growth, compression of neighboring organs.