Collagenous proteins are a major constituent of all extracellular matrices. Traditionally, the role attributed to collagen has been a structural one. During the past 10 years, however, it has become obvious that collagen comprises a large heterogeneous class of molecules, some with the structural properties classically attributed to “collagen,” but numerous others with additional properties. Vertebrates contain at least 15 different types of collagen. These are found in unique tissue-specific patterns, arise during development in defined temporal and spatial patterns, and exhibit different functional properties. Also, collagens have been shown to be involved, either directly or indirectly, in cell attachment and differentiation, as chemotactic agents, as antigens in immunopathological processes, and as the defective component in certain pathological conditions. Thus, in addition to their structural roles, collagens potentially have numerous developmental and physiological functions, many of which remain to be elucidated.