Partial reduction of molecular oxygen can generate reactive oxygen species (ROS), including the hydrogen peroxide, and the free radicals superoxide and hydroxyl. The formation of ROS is a feature of many degenerative diseases, such as atherosclerosis and neurodegeneration, Organisms contain a battery of defense mechanisms to prevent the formation of ROS, to scavenge them, and to repair the damage they cause. Free radicals are also involved in signal transduction pathways. For example, the free radical nitric oxide is involved in signal transduction in both the cardiovascular and central nervous systems. The interplay between nitric oxide and ROS has been a major focus of recent studies, as nitric oxide is an efficient radical scavenger. However, in some cases, such as in the formation of peroxynitrite from nitric oxide and superoxide, the product is potentially more deleterious that the parent radicals. This review describes the major chemical species involved in oxidative stress and free radical biochemistry, and gives a brief overview of their role in pathological conditions.