In the 10 years since its definition and characterization, TGF-β has come to be accepted as the prototypical multifunctional peptide growth factor. Attempts to understand its mechanism of action on cells have resulted in conceptual advances in knowledge of the contextual nature of growth factor activities. These have included modulatory effects both of sets of growth factors and of the cellular environment, including extracellular matrix (Sporn and Roberts, 1990; Nathan and Sporn, 1991). Because of the breadth of the cellular targets and activities of TGF-β it is impossible to be comprehensive in review of its physiological roles. Rather, we will cover selected aspects with emphasis on physiological roles which might ultimately find clinical application. As much as possible, we will consider in vivo roles of TGF-β in the context of the inherent complexity of cell-matrix and cell-cell interactions characteristic of mixed cell populations, tissues, or organs.