This book is a scientific biography of Louis Harold (“Hal”) Gray, FRS (1905–65), a pioneer in the field of radiobiology. Although radiobiology is a relatively obscure field of study, it is nonetheless extremely significant because it is the foundation of radiotherapy. The work done by Hal Gray was also crucial in ensuring that radiography would be a risk-free procedure for the hundreds of millions of people whose X-ray pictures have been taken. This was made possible as a direct result of Gray’s contributions. Both the development of radiobiology and Hal’s life are examined in this book, which provides some very interesting new perspectives on both topics. It includes a great deal of one-of-a-kind biographical material that Hal’s contemporaries, many of whom have passed away since the book was written, made available to the author over the course of the past 35 years. It is described how Hal’s unconventional high school, Christ’s Hospital, as well as his strong moral beliefs, have been significant influences on his life and his academic pursuits. However, his life was not merely a peaceful existence spent behind the walls of a university. Its highs and lows included occurrences that were not dissimilar to those that might have been found in a Hollywood drama. This book, which is the first full-length biography of Hal, is written for anyone who is interested in the history of radiobiology and radiotherapy. This includes people who do not have a scientific background but enjoy reading books of this genre.