The main components of basement membranes are collagen IV, laminin, entactin/nidogen, and proteoglycans. In the past few years, new basement membrane components have been discovered, including new collagen i.v. alpha chains, new laminin alpha, beta, and gamma chains, and new proteoglycans. Most of these are expressed in the kidney, a virtual treasure chest of molecularly and functionally distinct basement membranes. One well characterized renal basement membrane is the glomerular basement membrane, the primary filtration barrier of the kidney. Immunohistochemical studies have shown not only that the glomerular basement membrane is molecularly distinct from other renal basement membranes, but also that as it develops, there is a complex series of defined transitions in the basement membrane components that are deposited there. This review summarizes these transitions and discusses their relevance in our understanding of the pathogenesis of renal disease.