Bone is a hierarchically structured fiber composite made of collagen fibrils, reinforced by embedded mineral nanoparticles. First, the structural organization of lamellar bone concentrically wrapped around a central blood vessel (osteons) was investigated using synchrotron X-ray diffraction. Using a specially developed texture analysis technique with a 1 m thin X-ray beam, it was shown that the mineralized collagen fibrils are spiraling around the central axis. This most likely imparts high extensibility to the structure and shows that strains inside the osteon are taken up by shear between the fibrils. Secondly, the deformation mechanism of fibrolamellar bone was investigated at 3 levels of structural hierarchy, by combining mechanical in-situ tensile tests with X-ray diffraction. Special tensile testing devices were designed for this study, enabling the simultaneous determination of the overall strain in native bone tissue, as well as the strains in collagen fibrils and mineral particles. It was shown that fibrils take up only a fraction of the total strain, while the partially mineralized interfibrillar matrix takes up the remaining strain by shearing. This highlights the mechanical importance of the interfibrillar matrix which until now did not attract much attention. Overall, the results of this thesis will improve the understanding of bone brittleness resulting from diseases and, in addition, point out design-principles for new synthetic composites.