Originally published in 1949 as part of the Cambridge Monographs on Physics series, this book provides an outline of the main phenomena and techniques of radioactivity as understood at the time. Moon focuses on the radioactive properties of artificially-made atomic nuclei, as well as both the processes which result in a changed atomic number and those that do not. This book will be of value to anyone with an interest in the history of nuclear physics and the state of research after WWII.