The TGF-betas are a remarkable set of peptides consisting of three highly homologous isoforms, TGF-beta 1, 2, and 3. Distinguished initially for their ability to inhibit the growth of most epithelial and hematopoietic cells and to regulate the production of extracellular matrix by mesenchymal cells, these peptides are now known to act via autocrine, paracrine, and endocrine modes to control a wide variety of developmental processes and to play key roles in the pathogenesis of many diseases including especially fibrotic diseases, parasitic diseases, autoimmune diseases, and carcinogenesis. The activity of these peptides is under tight control by processes including regulation of the expression of the isoforms and their receptors and of the trafficking and activation of their latent forms.