Signalling by members of the transforming growth factor β (TGF-β)superfamily depends on a group of intracellular signalling molecules termed Smads. R-Smads are phosphorylated by the Ser/Thr kinase receptors to which these ligands bind; the R-Smads can then heterodimerize with a Co-Smad, enter the nucleus and regulate gene expression. In Cell Science at a Glance (seep. 3355 + poster), Aristidis Moustakas surveys the Smad signalling network, detailing its cytoplasmic and nuclear aspects, as well as the regulatory mechanisms that operate.