This book is a social-intellectual biography of the Belgian physicist Léon Rosenfeld (1904–1974). Rosenfeld was at the center of modern theoretical physics and he became the Danish physicist Niels Bohr's right-hand man. Rosenfeld was a perceptive, polyglot cosmopolitan, whose life crossed those of many important people in many countries. He was also a strong integrated personality capable of performing exotic calculations at one moment, while disentangling subtle philosophical questions or intervening in a political discussion the next — all at the highest level. This book aims at a broad audience interested in contextual history of twentieth century physics. No specialized knowledge of physics is required to read it.