Materials formed by organisms, also known as biological materials, exhibit outstanding structural properties. The range of materials formed in nature is remarkable and their functions include support, protection, motion, sensing, storage, and maintenance of physiological homeostasis. These complex materials are characterized by their hierarchical and composite design, where features with sizes ranging from nanometers to centimeters provide the basis for the functionality of the material. Understanding of biological materials is, while very interesting from a basic research perspective, also valuable as inspiration for the development of new materials for medical and technological applications. In order to successfully mimic biological materials we must first have a thorough understanding of their design. As such, the purpose of the characterization of biological materials can be defined as the establishment of relationships that relate the three dimensional material structure of a material to its function. By correlating structure with function we have the opportunity to apply biological design principles in alternative contexts for the development of materials with novel properties. In this thesis, several aspects of multiscale biological systems have been investigated and new research methods for automated Rietveld refinement and diffraction scattering computed tomography developed. The composite nature of biological materials was investigated at the atomic scale by looking at the consequences of interactions between mineral and the organic matrix in biomineralized calcite. High resolution powder diffraction was used to study how calcite in chalk, coccoliths, and mollusk shell is affected by the co-existent organic matrix. The calcified attachment organ in the saddle oyster, Anomia simplex serves as a brilliant example of biological design. We investigated the architecture of A. simplex and found that an advanced hierarchical biomineralized structure acts as the interface between soft musculature and a stiff substrate, thus securing underwater attachment. In bone, the mechanical properties of the material are defined by the organization of mineralized collagen fibrils. In this project, advanced synchrotron scattering techniques were used to investigate three aspects of mineralized collagen in bone: The growth of mineral particles in long bones, the orientation and microstructure of mineral in cortical bone, and the nanoscale response of bone in compression. Lastly, a framework for the investigation of biological design principles has been developed. The framework combines parametric modeling, multi-material 3D-printing, and direct mechanical testing to efficiently screen large parameter spaces of biological design. We used the framework to investigate several features of nacre design.