The collagens represent a large family of modular proteins which are located in the extracellular matrix. They all share a triple-helical segment of variable length (100–450 nm) but differ considerably in the size and nature of their globular domains. The various collagen types I-XIX known so far in vertebrates are adjusted to different functions including the formation of fibrils, microfibrils and networks. Their biological role is now increasingly studied by genetic approaches.