Reactive oxygen species (ROS) play a crucial role in various cellular functions, including cell homeostasis, inflammation, immunity, and carcinogenesis. They react preferentially with specific atoms in macromolecules, leading to both inhibitory and activating effects on proteins, DNA mutagenesis, and gene transcription. ROS are produced by multiple sources, including NADPH oxidases, mitochondria, and nitric oxide synthases, and their production is regulated by various checkpoints. ROS are also catabolized by antioxidant systems, such as superoxide dismutases and glutathione reductase, and can be repaired through mechanisms like nucleotide excision repair and base excision repair. In the immune system, ROS are involved in both innate and adaptive immunity, influencing the migration, activation, and function of immune cells. They are essential for the killing of pathogens by phagocytes and contribute to the regulation of lymphocyte function. However, excessive ROS production can lead to immunosuppression and contribute to tumor progression. Understanding the mechanisms of ROS production and action, as well as their translation into medical advances, remains a significant challenge.Reactive oxygen species (ROS) play a crucial role in various cellular functions, including cell homeostasis, inflammation, immunity, and carcinogenesis. They react preferentially with specific atoms in macromolecules, leading to both inhibitory and activating effects on proteins, DNA mutagenesis, and gene transcription. ROS are produced by multiple sources, including NADPH oxidases, mitochondria, and nitric oxide synthases, and their production is regulated by various checkpoints. ROS are also catabolized by antioxidant systems, such as superoxide dismutases and glutathione reductase, and can be repaired through mechanisms like nucleotide excision repair and base excision repair. In the immune system, ROS are involved in both innate and adaptive immunity, influencing the migration, activation, and function of immune cells. They are essential for the killing of pathogens by phagocytes and contribute to the regulation of lymphocyte function. However, excessive ROS production can lead to immunosuppression and contribute to tumor progression. Understanding the mechanisms of ROS production and action, as well as their translation into medical advances, remains a significant challenge.