Osteoporosis, one of the most important disorders associated with aging, needs active prevention. Estrogen therapy prevents the early phase of bone loss and decreases the incidence of subsequent osteoporosis related fractures. All estrogens are probably not equally effective against post menopausal bone loss. In USA conjugated equine estrogens are the most used. In Europe estrogen therapy is usually applied with 17 beta estradiol with different ways of administration. If estrogen therapy cannot be used, there are limited datas about the use of bisphosphonates or calcitonins in the prevention of post menopausal bone loss, but not still any data about the subsequent incidence of fractures. In all cases, calcium supplementation is effective.