Aerobic organisms possess antioxidant defense systems that deal with reactive oxygen species (ROS) produced as a consequence of aerobic respiration. Reactive oxygen is related to both, the arrest of growth and the start of cell differentiation. Low concentrations of reactive oxygen intermediates may be beneficial or even indispensable in processes such as intracellular messaging and defense against micro-organisms, but higher amounts of active oxygen may be harmful to cells and organisms. A wide array of non-enzymatic and enzymatic antioxidant defenses exists, including superoxide dismutase (SOD), glutathione peroxidase (GPX) and catalase (CAT). We describe their main characteristics and how these antioxidant enzymes work together against active oxygen. Small deviations from their physiological values may have a dramatic effect on the resistance of cells to oxidative damage to lipids, proteins and DNA. Consequently, toxic oxygen play a role in aging process as well as in a number of human diseases that we list in this review.