The transforming growth factor beta (TGF-beta) superfamily of dimeric polypeptide growth factors regulate cell growth and differentiation and play important roles in embryonic development and the immune response. The molecular cloning of three types of high affinity cell-surface receptors (type I, type II and type III) for these growth factors provide insights into their poorly understood mechanisms of action. Here we focus on the structure of these receptors and on the complex interactions which occur between different receptors that are necessary for ligand-induced signal transduction. The type I and type II receptors are both related transmembrane serine/threonine kinase receptors which may hetero-oligomerize to transduce proper signals. The type III receptor modulates the binding of ligands to the signaling complex of type I and type II receptors. These findings suggest that serine/threonine phosphorylation and heteromeric receptor interactions are important elements of TGF-beta induced signaling.