Free radicals are an integral part of metabolism and are formed continuously in the body. Many sources of stress heat, irradiation, hyperoxia, inflammation and any increases in metabolism including exercise, injury, and even repair processes lead to increased production of free radicals and associated reactive oxygen or nitrogen species (ROS/RNS). Evidence is accumulating that free radicals have important functions in the signal network of cells, including induction of growth and apoptosis and as killing tools of immunocompetent cells. Endogenous and nutritional antioxidant systems have to be adjusted to ensure adequate removal of radicals during stress to prevent damage to membranes, proteins, or nucleic acids. Excessive stress will induce DNA damage in the form of oxidized nucleosides, strand breaks, or DNA-protein crosslinks. Possible consequences of DNA damage are repair, apoptosis/necrosis, or defective repair leading to DNA sequence alterations and possibly to the development of cancer or, in case of mitochondrial DNA, to metabolic dysfunction. Excessive exercise will also induce DNA damage in peripheral leukocytes. The good message is that moderate stress in form of regular exercise/training may have protective effects against exercise-induced DNA damage. Up-regulation of endogenous antioxidant defense systems and complex regulation of repair systems such as heat shock proteins (HSP 70, HSP 27, HO 1) are seen in response to training and exercise. Up-regulation of antioxidants and modulation of the repair response may be mechanisms by which exercise can beneficially influence our health. Massive intervention into the redox state by pharmaceutical doses of exogenous antioxidants should be regarded with caution due to the ambiguous role of free radicals in regulation of growth, apoptosis, and cytotoxicity by immunocompetent cells.