All members of the sulphotransferase (SOT, EC 2.8.2.-) protein family use 3'-phosphoadenosine 5'-phosphosulphate (PAPS) as the sulphuryl donor and transfer the sulphonate group to an appropriate hydroxyl group of several classes of substrates. These enzymes have highly conserved domains and can be found in eubacteria and eukaryotes. In mammals, sulphate conjugation catalysed by SOTs constitutes an important reaction in the transformation of xenobiotics, and in the modulation of the biological activity of steroid hormones and neurotransmitters. In plants, sulphate-conjugation reactions seem to play an important role in plant growth, development, and adaptation to stress. To date only a few plant SOTs have been characterized in detail. The flavonol 3- and 4'-SOTs from Flaveria species (Asteraceae), which catalyse the sulphonation of flavonol aglycones and flavonol 3-sulphates, respectively, were the first plant SOTs for which cDNA clones were isolated. The plasma membrane associated gallic acid SOT of Mimosa pudica L. pulvini cells may be intrinsic to signalling events that modify the seismonastic response. In Brassica napus L. a SOT catalyses the O-sulphonation of brassinosteroids and thereby abolishes specifically the biological activity of 24-epibrassinolide. The fully sequenced genome of Arabidopsis thaliana Heynh. contains in total 18 genes that are likely to encode SOT proteins based on sequence similarities of the translated products with an average identity of 51.1%. So far only one SOT from A. thaliana (At5g07000) was functionally characterized: the protein was shown to catalyse the sulphonation of 12-hydroxyjasmonate and thereby inactivate excess jasmonic acid in plants. The substrates and, therefore, the physiological roles of SOTs are very diverse. By using the numerous informative databases and methods available for the model plant A. thaliana, the elucidation of the functional role of the SOT protein family will be accelerated.