Reactive oxygen species (ROS) and reactive nitrogen species (RNS) balance is critical in maintaining cellular homeostasis. Excessive levels of ROS (O2⨪, •OH, and H2O2) and/or RNS (ONOO−) affect the redox homeostasis, inducing oxidation of cellular nucleic acids, proteins, and lipids. The cells activate several antioxidant systems to maintain the intracellular redox equilibrium, including an enzymatic system (ascorbate peroxidase, glutathione peroxidase, peroxisomal catalase, and SODs) that works in concert with other nonenzymatic proteins (glutaredoxins, metallothionein, peroxiredoxins, and thioredoxins) and an nonenzymatic system (ascorbate, carotenoid, glutathione, melatonin, and tocopherol). In addition, autophagy is a very sensitive antioxidant system. NOXs = NADPH oxidases; cys-SH = cysteine-SH.