Transforming growth factor-β (TGF-β) is an important cytokine in glomerular disease. Its major role may be to mediate extracellular matrix deposition, by both increasing the synthesis of matrix components and by reducing their degradation. Strong evidence supports the functional role for TGF-β in mesangial matrix expansion. However, TGF-β may also have other important functions in the glomerulus, including the regulation of cell proliferation, hypertrophy, and survival (apoptosis), as well as modulation of the local and systemic immune response.