This review discusses the mechanisms and measurement of DNA damage caused by oxygen-derived species in mammalian systems. Oxygen-derived species, such as hydroxyl radicals (·OH), superoxide radicals (O₂⁻), and hydrogen peroxide (H₂O₂), can cause DNA damage through various mechanisms. These species can directly react with DNA, leading to chemical modifications, or indirectly through the activation of nucleases, which can cleave DNA strands. The damage can be measured by observing the patterns of chemical modifications and the products formed. Oxidative stress, caused by an imbalance between reactive oxygen species and antioxidant defenses, can lead to DNA damage, which is often measured as single-strand breaks, double-strand breaks, or chromosomal aberrations. The mechanisms of DNA damage by oxidative stress include the formation of ·OH radicals, which can cause extensive damage to DNA, and the activation of nucleases, which can cleave DNA strands. These two mechanisms are not mutually exclusive and can occur simultaneously. The review also discusses the role of reactive oxygen species as mutagens and carcinogens. Oxidative stress can lead to mutations in DNA, which can contribute to cancer development. The review highlights the importance of understanding the mechanisms of DNA damage and the role of reactive oxygen species in carcinogenesis. The review also covers the characterization and genetic effects of chemical changes in DNA caused by reactive oxygen species. These changes include the formation of various adducts and modifications to DNA bases, such as 8-hydroxyguanine and thymine glycol. These modifications can lead to mutations and are often measured using techniques such as HPLC and gas chromatography/mass spectrometry. The review concludes with a discussion on the measurement of base-derived products as a probe for the mechanism and extent of DNA damage. These measurements can provide insights into the role of reactive oxygen species in DNA damage and the mechanisms of carcinogenesis. The review emphasizes the importance of understanding these processes for the development of strategies to prevent and treat DNA damage and cancer.This review discusses the mechanisms and measurement of DNA damage caused by oxygen-derived species in mammalian systems. Oxygen-derived species, such as hydroxyl radicals (·OH), superoxide radicals (O₂⁻), and hydrogen peroxide (H₂O₂), can cause DNA damage through various mechanisms. These species can directly react with DNA, leading to chemical modifications, or indirectly through the activation of nucleases, which can cleave DNA strands. The damage can be measured by observing the patterns of chemical modifications and the products formed. Oxidative stress, caused by an imbalance between reactive oxygen species and antioxidant defenses, can lead to DNA damage, which is often measured as single-strand breaks, double-strand breaks, or chromosomal aberrations. The mechanisms of DNA damage by oxidative stress include the formation of ·OH radicals, which can cause extensive damage to DNA, and the activation of nucleases, which can cleave DNA strands. These two mechanisms are not mutually exclusive and can occur simultaneously. The review also discusses the role of reactive oxygen species as mutagens and carcinogens. Oxidative stress can lead to mutations in DNA, which can contribute to cancer development. The review highlights the importance of understanding the mechanisms of DNA damage and the role of reactive oxygen species in carcinogenesis. The review also covers the characterization and genetic effects of chemical changes in DNA caused by reactive oxygen species. These changes include the formation of various adducts and modifications to DNA bases, such as 8-hydroxyguanine and thymine glycol. These modifications can lead to mutations and are often measured using techniques such as HPLC and gas chromatography/mass spectrometry. The review concludes with a discussion on the measurement of base-derived products as a probe for the mechanism and extent of DNA damage. These measurements can provide insights into the role of reactive oxygen species in DNA damage and the mechanisms of carcinogenesis. The review emphasizes the importance of understanding these processes for the development of strategies to prevent and treat DNA damage and cancer.