Of the fractures linked with osteoporosis, hip fractures are most important in terms of death, functional dependence, and social cost. Reduction of these adverse outcomes depends on preventing hip fractures, which are caused by falling on a biomechanically compromised proximal femur. Skeletal strength depends on the amount of bone and its structural arrangement, whereas the traumatic load on the proximal femur is dictated by the orientation of the fall and other factors. One or several of these underlying mechanisms account for the various risk factors that have been identified for hip fracture, including greater incidence rates among women than men and among white that nonwhite populations. However, the exponentially increasing incidence of hip fractures with age indicates that a complex interplay of factors determines the risk of fracture in each person. The multifactorial nature of the problem suggests that hip fractures will be difficult to prevent even as the fractures and their associated costs continue to rise with the increased number of older people. However, the variability of hip fracture rates within high risk populations in Europe and North America reflects the existence of other powerful risk factors that, if identified, might provide the basis for more effective interventions.