Members of the transforming growth factor‐β (TGF‐β) superfamily bind to two different serine/threonine kinase receptors, i.e. type I and type II receptors. Upon ligand binding, type I receptors specifically activate intracellular Smad proteins. R‐Smads are direct substrates of type I receptors; Smads 2 and 3 are specifically activated by activin/nodal and TGF‐β type I receptors, whereas Smads 1, 5 and 8 are activated by BMP type I receptors. Nearly 30 proteins have been identified as members of the TGF‐β superfamily in mammals, and can be classified based on whether they activate activin/TGF‐β‐specific R‐Smads (AR‐Smads) or BMP‐specific R‐Smads (BR‐Smads). R‐Smads form complexes with Co‐Smads and translocate into the nucleus, where they regulate the transcription of target genes. AR‐Smads bind to various proteins, including transcription factors and transcriptional co‐activators or co‐repressors, whereas BR‐Smads interact with other proteins less efficiently than AR‐Smads. Id proteins are induced by BR‐Smads, and play important roles in exhibiting some biological effects of BMPs. Understanding the mechanisms of TGF‐β superfamily signalling is thus important for the development of new ways to treat various clinical diseases in which TGF‐β superfamily signalling is involved.