Basement membranes contain a number of intrinsic macromolecular components which are unique to these structures and which cooperatively assemble into specific heteropolymeric matrices. Type IV collagen triple helical monomers bind together at their amino-terminal, carboxy-terminal, and lateral domains to form a lattice-like array. Laminin, in a two-step-process, binds to itself at its terminal globular domains to form polymers and also binds collagen at two distinct sites along the collagen chain. Heparan sulfate proteoglycan has been found to bind both collagen and laminin, suggesting a reversible crosslinking function. On the basis of the data derived from self-association studies, it is possible to begin considering models for the assembly and structure of these ubiquitous matrices.