Free radicals, including reactive oxygen species (ROS) and reactive nitrogen species (RNS), play a crucial role in various physiological conditions and are implicated in numerous diseases. They are generated from both endogenous and exogenous sources, such as mitochondria, peroxisomes, endoplasmic reticulum, pollution, alcohol, tobacco smoke, heavy metals, and certain drugs. Free radicals can damage important biological molecules like nucleic acids, lipids, and proteins, leading to oxidative stress and various diseases such as diabetes, neurodegenerative disorders, cardiovascular diseases, respiratory diseases, cataracts, and cancers. ROS and RNS have dual roles, being beneficial at low levels but toxic at higher concentrations. They contribute to oxidative and nitrosative stress, which can damage biomolecules and lead to disease. The review discusses the chemistry, formation, sources, and molecular targets of free radicals, highlighting their role in disease pathogenesis. Free radicals are involved in various diseases, including diabetes, neurodegenerative disorders, cancer, cardiovascular diseases, cataracts, rheumatoid arthritis, asthma, and others. The review also covers the sources of free radicals, their molecular targets, and their role in disease development.Free radicals, including reactive oxygen species (ROS) and reactive nitrogen species (RNS), play a crucial role in various physiological conditions and are implicated in numerous diseases. They are generated from both endogenous and exogenous sources, such as mitochondria, peroxisomes, endoplasmic reticulum, pollution, alcohol, tobacco smoke, heavy metals, and certain drugs. Free radicals can damage important biological molecules like nucleic acids, lipids, and proteins, leading to oxidative stress and various diseases such as diabetes, neurodegenerative disorders, cardiovascular diseases, respiratory diseases, cataracts, and cancers. ROS and RNS have dual roles, being beneficial at low levels but toxic at higher concentrations. They contribute to oxidative and nitrosative stress, which can damage biomolecules and lead to disease. The review discusses the chemistry, formation, sources, and molecular targets of free radicals, highlighting their role in disease pathogenesis. Free radicals are involved in various diseases, including diabetes, neurodegenerative disorders, cancer, cardiovascular diseases, cataracts, rheumatoid arthritis, asthma, and others. The review also covers the sources of free radicals, their molecular targets, and their role in disease development.