Oxidative stress is one of the major symptoms accompanying physiological functions and numerous diseases. Oxidants and free radicals are connected with diseases such as cancer, diabetes, infectious, cardiovascular and neurodegenerative diseases and also in the aging process itself. Therefore, comprehensive knowledge of the underlying mechanisms of oxidant and free radical production and the defence against this permanent threat to cells is important for the understanding of many physiological and pathophysiological processes. This book overviews the basic mechanisms of oxidant formation and antioxidant defences. In addition, the formation of oxidants and their detoxification, the damage of macromolecules and the major repair and removal systems will be highlighted.