SummaryFree radicals are any molecules having an odd number of electrons. These molecules are highly reactive and can be generated as byproducts of normal metabolism as well as by exposure to a number of environmental factors including drugs, radiation and air pollutants. Due to the ubiquity of molecular oxygen, the oxygen metabolites superoxide anion, hydrogen peroxide, and the hydroxyl radical are frequently involved in both beneficial and detrimental free radical reactions. Intracellular enzymes and radical scavengers help to protect against tissue damage by these reactive metabolites. The extent of free radical damage to tissue depends on the nature of the radical produced and its site of generation.