Once scientists discovered radioactivity, they soon recognized its dangers. However, the assessment of radioactive risk to humans is still the subject of heated debate. In this book, the authors present a summary of past and recent scientific work on estimating radioactivity in the anthroposphere and the radioactive contamination of human beings. They describe the various sources of exposure to ionizing radiation (external, internal and dietary) and estimate the irradiation doses suffered by humans under various conditions (both naturally occurring for the general public and professionals, and from accidents). The harmful effects of ionizing radiation at various biological levels (molecular, cellular and tissue) and health effects at the individual level are also discussed. Particular focus will be placed on radiation-induced occupational illnesses. The relationship between doses of ionizing radiation and its harmful effects is discussed with reference to high, medium and low doses. The controversies on this subject are outlined, and international and French regulatory values are provided. Finally, an estimate of the radioactive risk for humans is suggested.