This is the first biography of Sir James Chadwick (1891-1974), best known as the discoverer of the neutron, for which he won the Nobel Prize. Chadwick's central role in the unfolding drama of nuclear physics is reflected in his publications and voluminous correspondence (many selections of which are included here) with other leading figures like Niels Bohr and Lord Rutherford. In the 1920s, Chadwick rose to become the operations director of the Cavendish Laboratory under Rutherford, and the discovery of the neutron came from an intense burst of work in 1930s, after a decade of disappointment. Chadwick's life was molded by great events, including both world wars (which carried him though internment camps and narrow escapes) and the development of the atom bomb. Indeed, during the Second World War, he was to become Britain's foremost authority on nuclear weaponry and chief British scientist on the Manhattan Project. His story thus offers unique insights into the behind-the-scenes activities of the U.S. and U.K. secret services and their quest for knowledge of German advances in the nuclear field. As an eye-witness account of some of the most dramatic discoveries and developments of the 20th century, Chadwick's biography is both gripping and insightful.