The transforming growth factor‐β (TGF‐β) superfamily includes more than 30 members which have a broad array of biological activities. TGF‐β superfamily ligands bind to type II and type I serine/threonine kinase receptors and transduce signals via Smad proteins. Receptor‐regulated Smads (R‐Smads) can be classified into two subclasses, i.e. those activated by activin and TGF‐β signaling pathways (AR‐Smads), and those activated by bone morphogenetic protein (BMP) pathways (BR‐Smads). The numbers of type II and type I receptors and Smad proteins are limited. Thus, signaling of the TGF‐β superfamily converges at the receptor and Smad levels. In the intracellular signaling pathways, Smads interact with various partner proteins and thereby exhibit a wide variety of biological activities. Moreover, signaling by Smads is modulated by various other signaling pathways allowing TGF‐β superfamily ligands to elicit diverse effects on target cells. Perturbations of the TGF‐β/BMP signaling pathways result in various clinical disorders including cancers, vascular diseases, and bone disorders. © 2001 Wiley‐Liss, Inc.