Within the past three decades accumulating evidence has pointed out the intermediancy of oxygen species in the chemical modification of natural and xeno-biotic compounds present in cells and tissues. A great number of international conferences as well as published papers, and books, all devoted to the reactive oxygen species and their role in the environment, indicate the importance and immediate interest of the problem. This chapter is not an exhaustive review, but presents the chemical mechanisms which have been shown to operate during damage to compounds of biological and industrial interest as well as in living systems. The possible medical implications of these species, chemical reactivity are also discussed.