This book presents the life and work of one of Wales’s most eminent scientists, Evan James Williams (1903–45). Williams’s background was humble: born in rural Cardiganshire, he made his way in the academic world by dint of a powerful intellect and vitally creative thinking. In the early twentieth century, he played a prominent part in the revolution in physics with the emergence of quantum physics, and he was an able experimentalist and brilliant theoretician who made notable contributions to atomic physics and the discovery of a new elementary particle. During World War II, he was instrumental in applying operational research to thwart the threat of German submarines in the Atlantic Ocean. This biography traces the course of Williams’s remarkable life and enables us to appreciate anew his many contributions to science and discovery.