Ionizing radiation is perhaps the most extensively studied human carcinogen. There have been a number of epidemiological studies on human populations exposed to radiation for medical or occupational reasons, as a result of protracted environmental exposures due to radiation accidents, or after atomic bombings. As a result of these studies exposure to ionizing radiation has been unambiguously linked to cancer causation. While cancer induction is the primary concern and the most important somatic effect of exposure to ionizing radiation, potential health risks do not only involve neoplastic diseases but also somatic mutations that might contribute to birth defects and ocular maladies, and heritable mutations that might impact on disease risks in future generations. Consequantly it is important we understand the long-term health risks associated with exposure to ionizing radiation.