[en] The review considers studies of the effects of ionizing radiation, UV, ultrasonics and chemical mutagens on DNA. Application of methods of chemical kinetics and other physical-chemical methods (EPR, chemiluminescence, spin labels, electron microscopy etc) to studying the mechanisms of DNA lesions is examined. Schemes of appearence of chemical and structural alterations in DNA are also analysed. Hypothesis of free-radical mechanism of radiation-induced DNA damage is discussed, along with the possible use of inhibitors of radical reactions as radioprotectors. Evidence supporting this hypothesis is presented