Bone strength is determined by bone mineral density, geometry of bone, microarchitecture of bone and quality of bone material. Recent data have shown that the prediction of bone strength can be greatly improved by including parameters of bone structure in the analysis. However, the relative importance of bone density and architecture in the etiology of bone fractures, an issue referred to as bone quality, is poorly understood. The challenge for the future will be to evaluate bone quality in vivo with the same or better accuracy than the invasive methods in use today.