Fluoride is an essential trace element. It is incorporated in calcified tissues. At doses lower than 1.5 mg/day, fluoride has a prophylactic action against dental caries by absorption on the enamel surface. At doses ranging from 15 to 25 mg/day, it is used as a therapeutic agent in adults, in the treatment of post-menopausal osteoporosis with vertebral crush fractures. At high doses, fluoride may induce skeletal fluorosis leading in children to pseudorachitism. In children, fluoride supplement has been recommended for several years, as a prophylactic agent against dental caries. It may be given either as dentifrices and toothwash solutions, easy to use and economic but contraindicated in very young children; or as tablets, sure and effective but requiring a daily intake; or as domestic salts supplemented with 250 mg/kg. Fluoride supplement may also be given through drinking water. Fluoride content is limited to 1.5 mg/l but some mineral waters contain high amounts of fluoride and may have a beneficial effect on bone mass. In order to prevent an excessive fluoride intake in children and in adults, different forms of fluoride supplements must not be associated.