A major portion of the morbidity, mortality, and social cost due to fall injuries is associated with age-related fractures, especially hip fractures. The risk of such injuries rises as the intensity of trauma increases and as bone resistance to fracture decreases. Both osteoporosis and trauma appear to be necessary but not sufficient causes of most age-related fractures. Future work may identify additional factors that help determine who, among the elderly who fall, is most likely to suffer a fracture. In the interim, however, prophylactic efforts must focus on avoiding falls and delaying the development of osteoporosis.