In this review, the basic processes responsible for radiation-induced changes in critical cell components and their biological consequences will be discussed. The chemical and physical alterations in biomolecules are mediated by free radicals and other reactive intermediates formed, following absorption of radiant energy, by ionization of proximal targets or the surrounding water molecules. Accumulation of free radical damage and its catalysis by various oxidants including quinones and other age pigments, metal ions, lipid peroxides, prostaglandins and components released from cells, increase with age. A cell's response to such damage depends upon environmental and inherited factors. DNA repair is an effective way to protect against radiation damage, but other constitutive or inducible defence mechanisms can also modify biological response, and these processes generally become less effective with age. Loss of fidelity with age of bio-feedback mechanisms including homeostasis, redox control, ion and metabolic regulation, which in turn affects cell growth and differentiation, energy efficiency, the immune system and general health, can be associated with free radical pathology. Current theories of aging will be examined including those involving wear-and-tear, genetic, metabolic, immunologic and biochemical factors. Ionizing radiation, as with other external stresses including UV, heat, chemotherapeutic agents, chemical carcinogens and tumor promoters, interact with nucleic acids, proteins and membrane phospholipids facilitating free radical-mediated oxidative damage. Appropriate antioxidant intervention, by inhibiting or reducing free radical toxicity, may offer protection against radiation, and alleviate or delay symptoms of aging and chronic disease.