Bone strength is determined by a number of important factors, including bone mass and bone shape. A reduction in bone strength is clearly related to fracture. Bone fragility results from a reduction in bone mass and density. If there is a reduction in the connectivity of bone and impact from a mechanical load occurs, bone will fracture. Rather than considering bone fragility as being the result of a reduced amount of bone, we recognize that bone fragility is the result of changes in the material and structural properties of bone. A better understanding of the contribution of each component of the material composition and structure, and how these interact to maintain whole bone strength is obtained by the study of metabolic bone diseases. Disorders of collagen, of mineral content composition and distribution, disorders of remodelling and other diseases produce abnormalities in the material composition and structure that lead to bone fragility.