Our world has been radioactive ever since! Humans are primarily exposed to natural radiation from the Sun, cosmic rays, and naturally-occurring radionuclides found in the Earth's crust. Besides the natural radioactivity, industries, which produce radioactive wastes during their normal operations or during their dismantling and decommissioning processes, do contaminate the environment through the release of radionuclides into the air, soil and water. Among them, nuclear power plants, NORM (Naturally Occurring Radioactive Materials) related industries, hospitals, radionuclide production facilities, uranium mining and other nuclear facilities, along with radioactive/nuclear disposal sites are a potential source of environmental contamination by emission/discharging of natural/artificial radionuclides through water, air and soil to the other environmental compartments like plants, animals and foods. In a word, everything that makes our existence! This is a comprehensive overview of some information on radiation in the environment and human exposure to radioactivity. This book highlights the sources, properties, behaviours, and biological and ecological effects of radioactivity from both natural and anthropogenic sources. The emphasis is on the environmental aspects of radionuclides and their eventual effects on biota, particularly humans.