In the 2 years following menarche, bone mass increases substantially; this is an important new finding concerning peak bone mass. Studies of twins have given significant insights into the effect of heredity on bone mass. Many determinants of bone mass, bone gain, and bone loss thus have been characterized for people of different ages. Techniques have been developed for measuring bone density and biomarkers, which are indicators of bone turnover, bone loss, and risk of fracture. Preventive therapy with estrogen has been validated, but the value of bisphosphonates remains to be confirmed. The administration of calcium and vitamin D reduces the incidence of hip fracture among elderly women in nursing homes. Curative etidronate therapy does not have a deleterious effect on bone histomorphometry.