Bone is organized in a hierarchical way that enables it to perform its mechanical, metabolic/endocrine, hematopoietic, and protective functions. At the nanoscale level, bone is a composite material composed of collagen, mineral, water, and a host of noncollagenous proteins. At the microstructural level, bone can be lamellar or nonlamellar, and can be formed either de novo, by direct apposition (primary), or by replacing existing bone (secondary). Macroscopically, bone is compact (cortical), with low porosity, or cancellous, with high porosity. This creates several surfaces from which bone can either be removed or added; these are termed the four skeletal envelopes (endocortical, periosteal, trabecular, intracortical). Because it is a dynamic living entity, bone also has its own special vascular supply and innervation. These features, together with the amount of bone and the quality and organization of the tissue, provide bone with its strength and fracture resistance.