From Wilhelm Conrad Röntgen's search for new kinds of radiation resulted the discovery of the natural radioactivity of uranium by Antoine Henry Becquerel in 1896. This event opened a new era of science that can be considered as the beginning of nuclear sciences. The findings of Pierre and Marie Curie of radium and polonium as useful radiation source subsequently opened the way to the first applications of radiation in medicine. Over the decades and still today radiation has found broad applications in many fields of science and technology. Not only chemistry, physics, biochemistry and medicine are using relevant applications of radioactivity. Biochemistry, agriculture, cosmo- and geochemistry, archaeology, geology also are fields where radiochemistry still is gaining importance. This book gives a survey about the history and modern aspects of radioactivity, new synthetic elements, and applications of radioactivity.