Because plants are static and live in a competitive and sometimes hostile environment, they have evolved mechanisms that protect them from abiotic and biotic stress. These mechanisms include detoxification and sequestration of xenobiotic compounds and of heavy metals, exploited in any phytoremediation process. However there must be a limit on the amount of pollutants that can be accumulated and detoxified without disrupting the normal plant metabolism and wellness. Enzymes involved in xenobiotics detoxification are often linked to the redox chemistry of the cell. The activities of cytochrome P450 monooxygenase, peroxidase and glutathione transferase have implications on the regulation of cellular redox status, closely related to mitochondrial respiration, also involved in maintaining the energy balance. Factors that disturb plant redox or energy status can thus affect both primary and secondary metabolism. For example, overloading a plant with high concentrations of xenobiotics requiring oxidation by P450 may compete with the normal functions of these enzymes. An increase in their activity may impose a major demand on both O2 and NAD(P)H pools. This could have significant effects on the overall redox and energy balance, thus compromising the primary and secondary metabolic processes in the plant and its survival. Molecules involved in conjugation of xenobiotics, like glutathione, also play a major role in normal plant metabolism. The presence of xenobiotic compounds induces the biosynthesis of glutathione transferases and thus an increased use of glutathione. Plant glutathione level and redox status are thus affected under such conditions, with probable implications on sulfur metabolism. A link does also exist between the degree of accumulation and tolerance of metals, the redox status and the antioxidant capacity of a plant. The lecture will highlight some well known or still unknown features, of utmost importance for a successful implementation of phytoremediation.