The interest in free-radical processes in living systems has increased exponentially during the last decade. The huge complexity of the evolved processes makes necessary the analysis of the problem from a fundamental point of view. Radicals are ubiquitous intermediates in a variety of ordinary biochemical reactions. Some of the radicals that are most abundantly produced in natural biochemical reactions are Reactive Oxygen Species (ROS) such as hydroxyl, hydroperoxyl and superoxide anion, and Reactive Nitrogen Species (RNS), such as nitrogen monoxide and peroxynitrite.
