The chemical structure of collagen, the most prevalent protein in skin and bone, is briefly reviewed. The regular repeating pattern of amino acid monomers (chiefly glycine, proline, hydroxyproline, and alanine) that promotes helix formation as well as providing the mechanism for attaching collagen molecules together into microfibrils and fibrils is described. The chemical structure of collagen in skin and bone are discussed in more detail. Collagen in the fiber network layer of skin is organized into long fiber bundles which form a dense, interlocking, three-dimensional network. The collagen in bone is a super-helix composed of three long chains, each of which consists of polypeptides arranged head-to-tail and having helices of their own. The organic phase of bone is composed of sheets of collagen fibrils and associated non-collagenous proteins. The non-collagenous proteins are thought to help maintain the steric, three-dimensional integrity of the fibril and to be necessary for deposition of the mineral phase in bone. This mineral phase (a type of hydroxyapatite) composes 75per thousand by weight of bone. The remaining 25per thousand of bone is made up of bone proteins.