Basement membranes are complex macromolecular structures which occupy the extracellular space between cells of different histologic types. Biochemically it is composed of Type IV collagen, several noncollagenous glycoproteins including laminin, fibronectin, GP-2 and PYS glycoprotein, and heparan sulfate. Morphologic changes are commonplace in a number of renal diseases. In diabetic glomerular disease, the basement membrane is markedly thickened but the biochemical basis has not been elucidated. In other disease-associated basement membrane changes, altered glycosylation of glycoprotein components has been described. The most important issue is the effect such alterations have on the interaction of basement membrane components and the function of the basement membrane.