You cannot hide from radioactivity. Even the book you are holding is slightly radioactive, less so if it is an e-book. Radioactivity - the breakdown of unstable atomic nuclei, releasing radiation - is a fundamental process in nature. It is a process that has been harnessed to provide wide and important applications in science, medicine, and energy production. But it remains much misunderstood - and feared, perhaps because nuclear radiation cannot be detected by human senses, and can undoubtedly do great harm if appropriate precautions are not taken. The traces of radioactive atoms in rock have allowed us to understand the nature and history of the Earth, in particular to date events in that history. Radioactive dating has been used for a variety of purposes, from archaeology to the dating of the Turin Shroud. In medicine, MRI scans and use of radioactive tracers have saved lives, and radioactivity is used widely in industry. And of course, it can provide a relatively cheap and clean source of energy, though the development of nuclear energy has been controversial and in recent times there have been increasing concerns about the dangers of nuclear terrorism. In this Very Short Introduction, Claudio Tuniz explains the nature of radioactivity and discuss its role in nature. Describing radioactivity in the stars and in the Earth, he also looks at its wide range of applications in biomedicine and in science, as well as the mechanisms of nuclear fission and fusion, and the harnessing of nuclear power.