The delivery of signals that control the growth of T cells is a key event for effective co-ordination of T-cell-dependent immune responses. It is now recognized that guanine nucleotide binding proteins play an important role in signal transduction by the T-cell receptor (TCR) and cytokine receptors. Here, Manolo Izquierdo Pastor, Karin Reif and Doreen Cantrell review the numerous recent advances in understanding how the p21ras guanine nucleotide binding protein couples the TCR to the T-cell signalling cascade.