This book describes the role of radioactivity in the living world and highlights the many aspects of the impact of ionizing radiation on biological systems. It addresses several topical issues, such as the countermeasures adopted to mitigate radiation-induced health risks during deep space missions, the concept of radiation hormesis, and recent biophysical models proposed to explain the radiation-adaptive responses. It critically approaches these intensely debated subjects and provides an up-to-date look at the ongoing studies on the use of radiation in nuclear clinical practice. With concrete examples, the book elucidates the results obtained from different types of radiotherapy as well as those that can be expected in the future. It comprises chapters dedicated to the analysis of social attitudes toward radioactivity and issues associated with the management of radioactive materials.