The mammalian glomerular capillary wall normally restricts the transmural passage of plasma proteins while offering little resistance to the filtration of water and small solutes. The basis for this selectivity has been explored extensively in recent years, through clearance measurements of endogenous (mainly albumin, transferrin, and immunoglobulins) and exogenous (horseradish peroxidase) proteins, and a variety of nonprotein polymers such as dextrans and polyvinylpyrrolidone. In conjunction with efforts to localize particulate and soluble tracers by high resolution ultrastructural techniques, such measurements have now made it possible to define the determinants of the glomerular filtration of macromolecules in terms of discrete structural barriers as well as such biophysical influences as hemodynamics and the molecular size- and charge-selective characteristics of the capillary wall. These experimental approaches have been aided greatly by the development of theoretical models that enable investigators to describe macromolecular filtration in terms of hydrodynamic principles applied to isoporous membranes. Although the initial models failed to consider the important role of membrane fixed negative-charge characteristics in influencing protein filtration, this shortcoming has led to the recent introduction of a theoretical model that also takes this factor into consideration. The aim of this brief review is to summarize these various theoretical approaches to the understanding of glomerular permselectivity and, wherever possible, to cite specific tests of these theories based on experimental studies in humans and animals.