Reactive oxygen species (ROS) are crucial in cellular functions, influencing cell homeostasis, inflammation, immunity, and carcinogenesis. They are produced by various enzymatic sources, including NADPH oxidases, mitochondrial respiratory chain, and other cellular pathways. ROS play a key role in host defense against pathogens and intermicrobial competition. Despite their importance, ROS are challenging to study due to their diverse sources, complex interactions, and the difficulty in quantifying and manipulating them. ROS also interact with reactive nitrogen species (RNS), such as nitric oxide, which share some properties but are distinct. ROS are involved in various cellular processes, including signal transduction, transcription, and DNA repair. They can both promote and prevent cell death, inflammation, and aging. ROS are produced by multiple cellular compartments, including mitochondria, phagocytes, and endosomes. Their regulation involves complex signaling pathways, including calcium signaling, phosphorylation, and small G protein activation. ROS can also be generated by inflammatory conditions, such as hypoxia, and by the activity of enzymes like nitric oxide synthases. ROS have significant roles in the immune system, particularly in innate immunity, where they are involved in the killing of pathogens by phagocytes. ROS are also crucial in the activation of transcription factors, DNA repair, and the regulation of immune cell functions. In lymphocytes, ROS contribute to signal transduction, cell proliferation, and immune responses. However, excessive ROS can be harmful, leading to oxidative stress and cellular damage. In cancer, ROS can both promote and inhibit tumor growth. Tumor cells often produce high levels of ROS, which can contribute to genomic instability and drug resistance. ROS also play a role in the tumor microenvironment, promoting angiogenesis and immunosuppression. Understanding the mechanisms of ROS production and their effects is essential for developing new therapeutic strategies in medicine. The study of ROS has evolved significantly, with new tools and techniques emerging to better understand their roles in cellular processes. These include genetically encoded redox-sensitive fluorescent proteins and advanced imaging techniques. The challenges in studying ROS remain, but ongoing research continues to uncover their complex roles in health and disease.Reactive oxygen species (ROS) are crucial in cellular functions, influencing cell homeostasis, inflammation, immunity, and carcinogenesis. They are produced by various enzymatic sources, including NADPH oxidases, mitochondrial respiratory chain, and other cellular pathways. ROS play a key role in host defense against pathogens and intermicrobial competition. Despite their importance, ROS are challenging to study due to their diverse sources, complex interactions, and the difficulty in quantifying and manipulating them. ROS also interact with reactive nitrogen species (RNS), such as nitric oxide, which share some properties but are distinct. ROS are involved in various cellular processes, including signal transduction, transcription, and DNA repair. They can both promote and prevent cell death, inflammation, and aging. ROS are produced by multiple cellular compartments, including mitochondria, phagocytes, and endosomes. Their regulation involves complex signaling pathways, including calcium signaling, phosphorylation, and small G protein activation. ROS can also be generated by inflammatory conditions, such as hypoxia, and by the activity of enzymes like nitric oxide synthases. ROS have significant roles in the immune system, particularly in innate immunity, where they are involved in the killing of pathogens by phagocytes. ROS are also crucial in the activation of transcription factors, DNA repair, and the regulation of immune cell functions. In lymphocytes, ROS contribute to signal transduction, cell proliferation, and immune responses. However, excessive ROS can be harmful, leading to oxidative stress and cellular damage. In cancer, ROS can both promote and inhibit tumor growth. Tumor cells often produce high levels of ROS, which can contribute to genomic instability and drug resistance. ROS also play a role in the tumor microenvironment, promoting angiogenesis and immunosuppression. Understanding the mechanisms of ROS production and their effects is essential for developing new therapeutic strategies in medicine. The study of ROS has evolved significantly, with new tools and techniques emerging to better understand their roles in cellular processes. These include genetically encoded redox-sensitive fluorescent proteins and advanced imaging techniques. The challenges in studying ROS remain, but ongoing research continues to uncover their complex roles in health and disease.