Type beta transforming growth factors (TGF beta s) are members of a large superfamily of related proteins, each of which plays a pivotal role in embryonic processes. The TGF beta s per se are at least five in number, though only three isoforms have been identified in mammals. Here we will review the evidence, taken from in vitro studies on bioactivity and histochemical localization of RNAs and encoded proteins in vivo, that TGF beta 1, beta 2 and beta 3 are involved in several mammalian developmental processes, including control of growth, differentiation, tissue inductions and morphogenesis.