Reactive oxygen species (ROS) are highly reactive derivatives that include free radicals such as superoxide anion radical (O2•−) and hydroxyl radical (HO•), as well as non-radical molecules like hydrogen peroxide (H2O2), peroxynitrite (ONOO−), and hypochlorous acid (HOCl). ROS play crucial roles in cellular signaling and regulation of various physiological processes, but their excessive production can lead to oxidative stress and cellular damage. The balance between ROS generation and antioxidant defense mechanisms is essential for maintaining cellular health. Antioxidants, including enzymatic and non-enzymatic antioxidants, help mitigate oxidative stress by counteracting ROS. However, the effects of antioxidants can be both beneficial and harmful, depending on the context and the specific conditions. The Special Issue "Cellular ROS and Antioxidants: Physiological and Pathological Roles" explores the dual nature of ROS and the complex effects of antioxidants. It highlights the beneficial roles of antioxidants in reducing the risk of chronic diseases, protecting against aging, improving immune function, and preventing cardiovascular and respiratory disorders. However, it also discusses the deleterious effects of antioxidants, such as pro-oxidant activity, increased cancer risk, interference with cell signaling, and compromise of immune function. The issue emphasizes the importance of understanding the compartmentalized action of ROS and antioxidants, as well as the need for further research to navigate the paradoxical effects of antioxidants. The articles in this Special Issue provide insights into the multifaceted roles of ROS and antioxidants, contributing to a deeper understanding of their physiological and pathological functions.Reactive oxygen species (ROS) are highly reactive derivatives that include free radicals such as superoxide anion radical (O2•−) and hydroxyl radical (HO•), as well as non-radical molecules like hydrogen peroxide (H2O2), peroxynitrite (ONOO−), and hypochlorous acid (HOCl). ROS play crucial roles in cellular signaling and regulation of various physiological processes, but their excessive production can lead to oxidative stress and cellular damage. The balance between ROS generation and antioxidant defense mechanisms is essential for maintaining cellular health. Antioxidants, including enzymatic and non-enzymatic antioxidants, help mitigate oxidative stress by counteracting ROS. However, the effects of antioxidants can be both beneficial and harmful, depending on the context and the specific conditions. The Special Issue "Cellular ROS and Antioxidants: Physiological and Pathological Roles" explores the dual nature of ROS and the complex effects of antioxidants. It highlights the beneficial roles of antioxidants in reducing the risk of chronic diseases, protecting against aging, improving immune function, and preventing cardiovascular and respiratory disorders. However, it also discusses the deleterious effects of antioxidants, such as pro-oxidant activity, increased cancer risk, interference with cell signaling, and compromise of immune function. The issue emphasizes the importance of understanding the compartmentalized action of ROS and antioxidants, as well as the need for further research to navigate the paradoxical effects of antioxidants. The articles in this Special Issue provide insights into the multifaceted roles of ROS and antioxidants, contributing to a deeper understanding of their physiological and pathological functions.