Bone is a two-phase porous composite material comprised primarily of collagen and mineral, which together provide its mechanical properties. The contribution of the mineral phase to bone's mechanical properties has dominated scientific thinking. Collagen's role has been underappreciated and not very well studied. However, there is evidence that changes in collagen content, or changes to inter- and intrafibrillar collagen cross-linking, can reduce the energy required to cause bone failure (toughness), and increase fracture risk. Although collagen may have less effect on bone's strength and stiffness than does mineral, it may have a profound effect on bone fragility. Collagen changes that occur with age and reduce bone's toughness may be an important factor in the risk of fracture in older women with low bone mass.