We live in a radioactive world. This fact poses some questions: where does it come from?; how much radioactivity is there?; how can it affect human health? The radioactive content surrounding us has different source terms depending on which radionuclides are considered. According to this, they can be classified into anthropogenic (or man-made) radionuclides and naturally occurring ones. In this book, the authors discuss the health hazards, medicinal benefit and uses of radionuclides.