Iron has a pivotal and dual role in free radical chemistry in all organisms. On the one hand, free Fe can participate in Fenton reactions and catalyze (‘catalytic Fe’) the generation of hydroxyl radical and other toxic oxygen species. On the other hand, Fe is a constituent of the antioxidant enzymes catalase, ascorbate peroxidase, guaiacol peroxidase, and ferro-superoxide dismutase. Protein Fe is Fenton inactive but can be released from proteins upon attack by activated oxygen. Healthy, unstressed plants avoid the interaction of catalytic Fe and peroxides by disposing of Fe in vacuoles and apoplast, by sequestering Fe in ferritin, and by having high levels of antioxidant enzymes and metabolites in most subcellular compartments. However, when plants are exposed to a variety of adverse conditions, including chilling, high light, drought and paraquat, oxidative stress ensues due primarily to the decrease in antioxidant defenses but also to the increase in free radical production mediated by catalytic Fe. The latter accumulates in many stressed plant tissues. Oxidative stress may lead to metabolic dysfunction and ultimately to plant cell death, so it needs to be estimated conveniently by quantifying the oxidation products of lipids (malondialdehyde and other cytotoxic aldehydes), proteins (total carbonyls, methionine sulfoxide, 2-oxohistidine), and DNA (8-hydroxyguanine, 5-hydroxycytosine). Protein oxidation appears to be a more sensitive and precocious marker than is lipid peroxidation, and DNA damage may also prove to be a useful marker for stress studies in plants.