The chapter discusses the role of oxygen and superoxide dismutases (SODs) in aerobic organisms, highlighting the paradoxical nature of oxygen's dual role as a vital energy source and a cytotoxic oxidant. Molecular oxygen (O₂) is relatively unreactive in its ground state but can form highly reactive species like superoxide radicals, hydrogen peroxide (H₂O₂), and hydroxyl radicals during metabolic processes. These reactive oxygen species (ROS) can cause significant cellular damage, leading to oxidative stress. Plants, in particular, face unique challenges due to their reliance on O₂ for respiration and photosynthesis, making them susceptible to environmental stresses that generate ROS. The chapter outlines the protective antioxidant defenses in aerobic organisms, including both enzymatic and nonenzymatic mechanisms. Enzymatic defenses, such as SODs, play a crucial role in scavenging ROS. SODs are metalloproteins that efficiently convert superoxide radicals to less harmful forms like water and molecular oxygen. The chapter details the different types of SODs (Cu/ZnSOD, MnSOD, and FeSOD) and their distribution in various cellular compartments. It also discusses the regulatory mechanisms that control SOD expression in response to oxidative stress, emphasizing the importance of maintaining a balance between ROS generation and removal. The chapter further explores the physiological functions of SODs, particularly in plants, where multiple forms (isozymes) of SOD exist. These isozymes have distinct cellular locations and responses to various environmental stresses, suggesting their specific roles in protecting against oxidative damage. The chapter concludes by highlighting the need for a deeper understanding of the molecular mechanisms underlying SOD regulation and expression, as well as the potential for engineering more oxidative stress-tolerant organisms.The chapter discusses the role of oxygen and superoxide dismutases (SODs) in aerobic organisms, highlighting the paradoxical nature of oxygen's dual role as a vital energy source and a cytotoxic oxidant. Molecular oxygen (O₂) is relatively unreactive in its ground state but can form highly reactive species like superoxide radicals, hydrogen peroxide (H₂O₂), and hydroxyl radicals during metabolic processes. These reactive oxygen species (ROS) can cause significant cellular damage, leading to oxidative stress. Plants, in particular, face unique challenges due to their reliance on O₂ for respiration and photosynthesis, making them susceptible to environmental stresses that generate ROS. The chapter outlines the protective antioxidant defenses in aerobic organisms, including both enzymatic and nonenzymatic mechanisms. Enzymatic defenses, such as SODs, play a crucial role in scavenging ROS. SODs are metalloproteins that efficiently convert superoxide radicals to less harmful forms like water and molecular oxygen. The chapter details the different types of SODs (Cu/ZnSOD, MnSOD, and FeSOD) and their distribution in various cellular compartments. It also discusses the regulatory mechanisms that control SOD expression in response to oxidative stress, emphasizing the importance of maintaining a balance between ROS generation and removal. The chapter further explores the physiological functions of SODs, particularly in plants, where multiple forms (isozymes) of SOD exist. These isozymes have distinct cellular locations and responses to various environmental stresses, suggesting their specific roles in protecting against oxidative damage. The chapter concludes by highlighting the need for a deeper understanding of the molecular mechanisms underlying SOD regulation and expression, as well as the potential for engineering more oxidative stress-tolerant organisms.