The connective tissue protein, collagen, is the most abundant protein in higher animals where it occurs primarily as extracellular, insoluble fibers. These fibers account for a large part of the organic mass of skin, tendon, blood vessels, bone, teeth, cornea, and vitreous humor. Collagen also provides the framework for most of the parenchymal organs, either in its fibrous form or organized in basement membranes. Because of the ubiquitous distribution and abundance of this structural protein it has been investigated by scientists working in a wide variety of disciplines and specialties ranging from such applied areas as suture production and leather manufacture to the more fundamental aspects of polymer chemistry and the structure-function studies of the protein chemist. In this article I discuss in some detail the structure and function of collagen. In particular I present agruments that (i) the insolubility of collagen fibers is primarily a consequence of covalent crosslinking; (ii) the covalent cross-links arise from modified amino acids which contain carbonyl groups; and (iii) the molecular packing of collagen monomers into the polymer (fibril) probably specifies which cross-links are formed.