The paradox of aerobic life, or the 'Oxygen Paradox', is that animals and plants cannot exist without oxygen, yet oxygen is inherently dangerous to their existence. The reductive environment of cells provides ample opportunities for oxygen to undergo unscheduled reduction events, yielding free radicals that catalyze lipid peroxidation. Oxidized lipids are constitutively present in higher organisms and, notably, their levels increase in response to a variety of stresses. Recent results suggest that products of non-enzymatic lipid peroxidation pathways, such as the isoprostanes/phytoprostanes in animals and plants, might have an evolutionarily ancient function in host defense.