Bone density and bone quality have been cited as primary contributors to bone strength. Although a precise definition of bone quality remains elusive, characteristics other than bone density, such as trabecular architecture, bone turnover, and mineralization of the bone matrix may be important factors in the pathophysiology of osteoporosis and the mechanisms that underlie fracture. Small increases in bone mineral density (BMD) have resulted in greater than expected reductions in the risk of fracture. Indeed, therapeutic increases in BMD account for only a small proportion of the reduction in fracture risk. This observation suggests that the size, shape, distribution, and composition of bone may have important effects on bone strength and, therefore, the risk of fracture.