Risk of osteoporotic fracture is dependent on peak bone mass achieved at skeletal maturity and subsequent bone loss. Accordingly, reduction in this risk can be achieved by maximizing the peak bone mass, preventing subsequent bone loss, or increasing bone density in patients with established osteoporosis. Recent studies in twins have shown that although peak bone mass is strongly genetically determined, it can be influenced by dietary calcium intake and physical activity. Bone mass gain is rapid during puberty, and delayed puberty may alter peak bone mass. Estrogen therapy can prevent bone loss in postmenopausal women. Bone densitometry can be used to select patients who are suitable for therapy and enhance compliance; however, screening is not justified. Treatment of established bone loss remains a problem, although recent studies of estrogen, vitamin D, and bisphosphonates are encouraging.