The incidence of osteoporotic fractures is at epidemic proportions in North America and will increase steadily as the population ages. Though people fall more often with age, most etiologic factors act through bone mass, and fracture risk at various sites has been found to increase exponentially with decline in bone mineral density (BMD). Thus research has focused on factors that affect BMD, particularly hormonal factors such as estrogen and calcitonin, resorption inhibitors, nutritional factors such as calcium and vitamin D, and physical activity.