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.