Recent discoveries in kidney research have given new insights into the molecular make-up of the glomerular filter and mechanisms of permselectivity. The identification of mutations in the genes for glomerular basement membrane type IV collagen has thus demonstrated the central role of the glomerular basement membrane as the structural skeleton of the glomerular capillary. Regional deterioration of this framework not only leads to proteinuria, but also to significant leakage of red blood cells into the urinary space. Tracer studies and the characterization of other glomerular basement membrane components, such as proteoglycans, have also emphasized the role of the glomerular basement membrane in the permselectivity process. However, more recent studies on nephrin, a key component of the slit diaphragm, as well as the podocyte and slit diaphragm-associated intracellular proteins, CD2-associated protein, podocin and alpha-actinin-4, have emphasized the role of the slit diaphragm as a central size-selective filtration barrier. These data have provided a completely new understanding of the mechanisms of proteinuria, both in inherited and acquired diseases. In this review, we present the recent progress made in the characterization of proteins that are important for glomerular permselectivity.