DNA, deoxyribonucleic acid, is crucial to life. With its triplet coding, DNA serves as the template for messenger RNA, and is therefore responsible for the myriad proteins that ensure the ongoing health and life of the current cell or organism. Future generations are similarly dependent on the DNA code, which cellular mechanisms help propagate. DNA strands must remain intact and undamaged to guarantee that the code functions with high fidelity. If DNA becomes covalently damaged, repair mechanisms can excise the affected region and replace it with the correct sequence. Indications of the biochemical transformation can be found among the excretion products of the cell or organism. However, the repair processes do not always work perfectly, and certain modifications escape the repair system. These covalent modifications may be of critical importance to the causes of disease states, including carcinogenesis. In addition, certain medications, known to generate covalent modifications of DNA, are used therapeutically in an effort to combat an observed disease state. These deliberately produced DNA modifications also need to be studied. In the review, the term DNA modification includes not only DNA adducts (xenobiotic compounds covalently bound to DNA) but also other aberrant forms of DNA, such as ring-opened products and dimers.