The composition of the extracellular matrix (ECM) varies depending on tissue location and developmental stage. Collagens as the main constituents determine its structure and play a major role in determining its function. Polymers of fibrillar collagens (I,II) form the backbone of many ECMs, whereas many minor collagens regulate or stabilize their structural properties. Changes in the minor collagens introduce subtle modifications in intermolecular interactions, which can modulate the morphology of the ECM and responses of the underlying or embedded cells. The functions of these modulating collagens is now being investigated by a number of biochemical, genetic and molecular approaches.