T here have been exciting recent advances in our understanding of the structural and molecular biology of the glomerular slit diaphragm, as described in a report in this issue of the JCI. These findings, combined with data on the permeability of the basement membrane and evidence that the endothelium may be a more important barrier than often supposed, are allowing a clearer understanding to emerge of how the 3 parts of the glomerular capillary wall jointly determine its functional properties.