The article reviews the role of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in chronic diseases and aging. ROS and RNS are produced endogenously and exogenously, and their levels can be elevated under various conditions, leading to oxidative stress. Oxidative stress is characterized by a shift towards the formation of more oxidants, resulting in damage to DNA, proteins, and lipids. The article discusses the chemistry of ROS and RNS, including superoxide radical, hydrogen peroxide, hydroxyl radicals, peroxyl radicals, nitric oxide, and peroxynitrite. It also covers the assessment of oxidative stress biomarkers and the mechanisms by which oxidative stress contributes to various diseases such as cancer, cardiovascular diseases, diabetes, neurological disorders, psychiatric disorders, and aging. The article highlights the importance of antioxidant enzymes (such as superoxide dismutases, catalase, and glutathione peroxidase) and low molecular weight antioxidants (such as vitamins C and E, flavonoids, carotenoids, melatonin, and ergothioneine) in mitigating the harmful effects of oxidative stress. Additionally, it explores the potential of certain antioxidants with weak prooxidant properties, such as flavonoids, as preventive agents against cancer. The article also discusses redox metal-based enzyme mimetic compounds and sirtuins as promising therapeutic targets for age-related diseases and anti-aging strategies.The article reviews the role of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in chronic diseases and aging. ROS and RNS are produced endogenously and exogenously, and their levels can be elevated under various conditions, leading to oxidative stress. Oxidative stress is characterized by a shift towards the formation of more oxidants, resulting in damage to DNA, proteins, and lipids. The article discusses the chemistry of ROS and RNS, including superoxide radical, hydrogen peroxide, hydroxyl radicals, peroxyl radicals, nitric oxide, and peroxynitrite. It also covers the assessment of oxidative stress biomarkers and the mechanisms by which oxidative stress contributes to various diseases such as cancer, cardiovascular diseases, diabetes, neurological disorders, psychiatric disorders, and aging. The article highlights the importance of antioxidant enzymes (such as superoxide dismutases, catalase, and glutathione peroxidase) and low molecular weight antioxidants (such as vitamins C and E, flavonoids, carotenoids, melatonin, and ergothioneine) in mitigating the harmful effects of oxidative stress. Additionally, it explores the potential of certain antioxidants with weak prooxidant properties, such as flavonoids, as preventive agents against cancer. The article also discusses redox metal-based enzyme mimetic compounds and sirtuins as promising therapeutic targets for age-related diseases and anti-aging strategies.