Within the body there is continual production of entities known as Reactive Oxygen Species (ROS). These include radical derivatives of oxygen that contain at least one unpaired electron and include species such as the superoxide and hydroxyl radicals. ROS also include nonradical derivatives of oxygen that are capable of oxidising biomolecules such as hydrogen peroxide, ozone and nitrogen dioxide. ROS are formed from processes such as the respiratory burst of phagocytes and from mitochondrial oxidative phosphorylation, so production is often increased by situations that elevate oxygen utilisation such as exercise. ROS may also act as “signalling” species within the body. Controlled production of ROS is therefore essential for normal cellular function and health, especially with respect to the functioning of the immune system. However, uncontrolled production of ROS can result in cell damage and death, the induction and propagation of inflammation and DNA damage. Thus, the body has evolved intricate and elaborate enzymatic and non–enzymatic antioxidant defences to control and buffer excess ROS production. In situations where the antioxidant defences are overwhelmed either due to their depletion, malfunction or simply due to excessive ROS bombardment, oxidative stress and oxidative damage are likely to occur.