Activins and inhibins, which were discovered by virtue of their abilities to stimulate or inhibit, respectively, the secretion of FSH, are members of the transforming growth factor-beta (TGFbeta) superfamily and exert a broad range of effects on the diffentiation, proliferation and functions of numerous cell types. Activins interact with two structurally related classes of serine/threonine kinase receptors (type I and type II). Inhibin antagonizes activin by binding to the proteoglycan, betaglycan, and forming a stable complex with and, thereby, sequestering type II activin receptors while excluding type I receptors. If betaglycan is present, inhibin can also antagonize those bone morphogenic proteins (BMPs) whose signaling is dependent upon access to type II activin receptors. Recent insights regarding the structures of ligands, receptors and their signaling complexes are providing the basis for the development of therapeutics capable of modulating fertility and numerous pathophysiologic processes.