Both "environmental chemistry" and metabolic biochemical reactions can constantly generate in vivo free radicals and other oxygen-derived species that can cause severe damage to almost all biomolecules, especially to DNA, proteins, and lipids. The superoxide anion has been shown to be the most readily generated and spread radical among organisms and it is a common intermediate of oxidative stress processes in the cells. The antioxidant defense system of superoxide dismutases (SOD) scavenges and minimizes the formation of this radical, and thus plays a major role in reducing cumulative oxidative damage in different cell compartments both in aerobic and anaerobic cells. In the cell, cytosol SODs are constitutively present and induced by many oxidative agents able to raise the superoxide concentrations. Presence of SODs, however, in extracellular cell-associated locations demonstrates how valuable they are in maintaining the integrity of cells against oxidative stress generated by the cell environment, particularly upon increased oxygenation. Because SODs have recently been found in Archaea, which are prokaryotes, sometimes living in extreme environments, even in anaerobic ones, these enzymes can be considered essential: they may have allowed the evolution of aerobic respiration starting from an ancient form of oxygen-insensitive life.