The book is concerned with features analysis of radiation damage to living matter at different levels of its organisation, beginning from the molecular level. The general theory of the mechanisms of biological action of radiation based on four logically connected principles of radiation biology is presented for the first time. The book contains abundant material on primary and secondary processes of the response of living systems to radiation, the mechanisms of direct and indirect action of radiation through radiolysis of water and lipids; a detailed consideration of the theory of targets (DNA and biological membranes) and the problems of high- and low-dose irradiation in the context of present-day. The systemic response of the cell to irradiation (oxidative stress, endogenous background radioresistance, damage and repair of biological membranes and DNA, systemic protection against radiation) is approximated. Presentation of these important problems is preceded by an up-to-date evaluation of some aspects regarding dosimetry of ionising radiation.