Plants adapt to environmental stresses through specific genetic responses. The molecular mechanisms associated with signal transduction, leading to changes in gene expression early in the stress response, are largely unknown. It is clear, however, that gene expression associated with acclimatory responses is sensitive to the redox state of the cell. Of the many components which contribute to the redox balance of the cell, two factors have been shown to be crucial in mediating stress responses. Thiol/disulphide exchange reactions, particularly involving the glutathione pool and the generation of the oxidant H2O2, are central components of signal transduction in both environmental and biotic stresses. These molecules are multifunctional triggers, modulating metabolism and gene expression. Both are able to cross biological membranes and diffuse or be transported long distances from their sites of origin. Glutathione and H2O2 may act alone or in unison, in intracellular and systemic signalling systems, to achieve acclimation and tolerance to biotic and abiotic stresses.