Glutathione S-transferases (GSTs) are abundant proteins encoded by a highly divergent, ancient gene family. Soluble GSTs form dimers, each subunit of which contains active sites that bind glutathione and hydrophobic ligands. Plant GSTs attach glutathione to electrophilic xenobiotics, which tags them for vacuolar sequestration. The role of GSTs in metabolism is unclear, although their complex regulation by environmental stimuli implies that they have important protective functions. Recent studies show that GSTs catalyse glutathione-depend-ent isomerizations and the reduction of toxic organic hydroperoxides. GSTs might also have non-catalytic roles as carriers for phytochemicals.