Recent studies have established that connective tissue contains several chemically and genetically distinct collagens. Here we discuss the nature of these proteins, their occurrence in different tissues and their involvement in disease processes. Types I, II and II collagens show some similarities indicating a common origin. Morphological studies using antibodies against the various collagens indicate that the different collagens occur in different anatomical structures. Alterations in the distribution of these collagens are seen in tissues in certain disease states and may cause the loss of normal tissue function.