TGF-β is now recognized as an important factor regulating normal mammary gland development as well as breast cancer. In development, TGF-β regulates branching morphogenesis and differentiation by acting on both epithelial and stromal cells. TGF-β also regulates apoptosis and matrix remodeling during involution at the end of the pregnancy cycle. TGF-β has biphasic effects on tumor progression, acting as a tumor suppressor in early stages of cancer and promoting invasion and metastasis at later stages. Furthermore, TGF-β may play a role in tumor progression through effects on the microenvironment. The tumor promoting effects of TGF-β may provide a therapeutic target for late stage breast cancer via TGF-βantagonists like the soluble receptors recently described. Future experiments will uncover the precise mechanisms of TGF-β action in development and neoplastic disease providing more opportunities for prevention and treatment of breast disease.