The process of biotransformation of toxic compounds in living organisms typically comprises two phases. The main complex of phase I is the P-450 cytochrome enzymatic system; in phase II it is glutathione S-transferase enzymes. In helminths the activity of the P-450 cytochrome system is reduced, wherefore compensatory changes of other components of biochemical protection, including the strengthening of the metabolic role of the enzymes of the second phase of biotransformation, can be hypothesized. Glutathione S-transferases may thus assume the key role in detoxification processes in helminths. Glutathione S-transferases are an evolutionarily ancient family of multifunctional enzymes which participate in detoxification of potentially dangerous exo- and endogenous molecules (carcinogens, drugs, peroxidation products, etc.) by catalyzing the conjugation of organic molecules with reduced glutathione. In the present review the current state of research on glutathione S-transferases in helminths has been analyzed, and information on the isolation and study of the isoenzyme spectrum of cytosolic glutathione S-transferases (cGST) of helminths and their hosts is given. The comparative analysis of the range of GST isoenzymes in different members of classes Trematoda, Cestoda, Nematoda was carried out. Both similarities and essential differences of cGST of parasites and their hosts are shown at the structural, biochemical and molecular levels. Data on the nomenclature and classification of the enzymes are also presented. It is demonstrated that studies of parasite-specific enzymatic systems contribute to the detection of the proteins playing a crucial role in the helminth survival in the host and are essential for the development of anthelmintic therapy.