From studies of the 'classical' components, models for the assembly and structure of an idealized basal lamina have been developed. In particular, the evidence supports the concept of enmeshed collagen and laminin polymers, in which nidogen/entactin acts as a bridge between these molecules and provides anchorage for diverse matrix components. Different basement membranes, however, possess different members of the basic basal lamina families, such as the newly described alpha 6 (IV) collagen, alpha 2 (merosin) laminin, and beta 3 laminin (in kalinin/nicein) chains. Even though these members share homologous domains and sequences, and are likely to share certain functions, they also possess unique characteristics that are expected to provide for basal lamina heterogeneity. A combination of genetic, recombinant and biochemical approaches are now being applied to elucidate the special roles of both old and new components.