Partially-reduced forms of dioxygen or "oxy-radicals" (superoxide, O2-/HO2; hydrogen peroxide, H2O2; hydroxyl radical X OH) and oxidants of comparable reactivity are implicated in an increasing number of physiological, toxicological, and pathological states. Transition metal catalysis is recognized as being integral to the generation and the reactions of these activated oxygen species. Factors such as pH and chelation govern the reactivity of the transition metals with dioxygen and "oxy-radicals" and therefore influence the apparent mechanisms by which oxidative damage to phospholipids, DNA, and other biomolecules is initiated. In biological systems the concentrations of redox-active transition metals capable of catalyzing these reactions appears to be relatively low. However, under certain conditions metal storage and transport proteins (ferritin, transferrin, ceruloplasmin, etc.) may furnish additional redox active metals.