Nephrotic syndrome results from varying injuries to the capillary wall of the glomerulus. The components of the capillary wall, including the endothelial cell, glomerular basement membrane and glomerular visceral epithelial cell all may be targets of injury and contribute to proteinuria. These mechanisms of injury include immune complexes, cytotoxins, abnormal protein deposition, metabolic abnormalities, reactive oxygen species, growth factors, hemodynamic stress, and genetic abnormalities. We review mechanisms of glomerular permselectivity, with focus on emerging new understanding of the functions of the glomerular visceral epithelial cell. The significance and consequences of proteinuria and possible pathogenic mechanisms and the effect of interventions in clinical renal disease on these factors are considered.