Plant cytosolic glutathione transferases (GSTs) belong to an ancient enzyme superfamily with multiple and diverse functions which are important in counteracting biotic and abiotic stress. GSTs catalyze the conjugation of xenobiotics and endogenous electrophilic compounds with glutathione (GSH), leading to their detoxification. GSTs not only catalyze detoxification reactions but they are also involved in GSH-dependent isomerization reactions, in GSH-dependent reduction of organic hydroperoxides, biosynthesis of secondary metabolites, and exhibit thioltransferase and dehydroascorbate reductase activity. The applications of ‘omics’ technologies have allowed the classification of GSTs and the study of their evolution and sequence diversity, while enzymology has provided powerful insights into their catalytic role. This review focuses on plant GSTs, and attempts to give an overview of the new insights into their catalytic function and biological role in biotic and abiotic stress tolerance mechanisms in plants.