Bones are designed to carry out their requirements effectively. One of these requirements is to resist fracture. Two other important requirements are to be stiff and to be light. Few theories of adaptive modeling distinguish modeling for adequate stiffness from modeling for adequate strength. Bones achieve their architecture partially through genetics, the rough form of the bone being laid down in the genes, and partially through response to normal loading. Normal loading rarely includes traumatic loading and bones are not usually well adapted to resist trauma, though they are probably well adapted to fatigue loading. Some aspects of architectural function, such as hollowness, are well understood. Some aspects, such as the need for uniform loading in impact, are less understood, and some, such as size effects, are only now beginning to be investigated.