S-Glutathionylation is the specific post-translational modification of protein cysteine residues by the addition of the tripeptide glutathione, the most abundant and important low-molecular-mass thiol within most cell types. Protein S-glutathionylation is promoted by oxidative or nitrosative stress but also occurs in unstressed cells. It can serve to regulate a variety of cellular processes by modulating protein function and to prevent irreversible oxidation of protein thiols. Recent findings support an essential role for S-glutathionylation in the control of cell-signalling pathways associated with viral infections and with tumour necrosis factor-(-induced apoptosis. Glyceraldehyde-3-phosphate dehydrogenase has recently been implicated in the regulation of endothelin-1 synthesis by a novel, S-glutathionylation-based mechanism involving messenger RNA stability. Moreover, recent studies have identified S-glutathionylation as a redox signalling mechanism in plants.