Phytoparasitic nematodes belong to the animal kingdom and are obligate parasites of almost all the plant species growing in the world. Both nematodes and plants are aerobic organisms as they require oxygen to produce energy for cellular activities. For this purpose, molecular oxygen is reduced to water by mitochondrial cytochrome oxidase in the vital process known as cellular respiration. Nevertheless, oxygen is a source of toxicity as it can be converted by single-electron reduction into highly reactive oxygen species, such as superoxide, hydrogen peroxide and hydroxyl radical, which are able to react with and damage all the biological molecules. Thus, both nematodes and plants have to control strictly the cellular level of such species in order to avoid cellular oxidative degeneration. Highly effective antioxidant systems have been specifically arranged by co-evolution of plants and nematodes. Antioxidant enzymes, such as superoxide dismutase, catalase, ascorbate and glutathione peroxidase, are highly active in the parasites and their hosts. Resistant responses may be provoked by a nematode-induced inhibition of such antioxidant enzymes which produces a local area of tissue necrosis thus restricting nematode development. Compatible response are allowed by maintaining low toxic oxygen species in cells surrounding nematode, which can exert its feeding action only in a non-hostile environment. Reaction of the plant to nematode attack implies marked variations of oxygen metabolism that are considered herein as critical for a successful development of the parasites.