The ultrastructural basis of glomerular permeability to protein molecules is reviewed. These studies have relied either on the intrinsic electron density of intravenously injected tracer molecules, or on the peroxidatic activity of a number of peroxidatic enzymes of varying molecular weight. The nature of the final filtration barrier in the glomerulus remains conjectural but two possibilities have been suggested: the glomerular basement membrane (a) is the sole filter and retards the passage of protein molecules in a fashion analogous to gel filtration, or (b) it acts as a coarse filter in series with a fine, filter located in the slit-pore complex. Correlative studies of mathematical models and physiological data tend to favor the latter possibility.