PAUL LANGEVIN (1872-1946) was a renowned French physicist, humanist, and educationalist.This biography by his son André, originally published in French in 1972 and here translated into English for the first time, is enhanced by numerous quotations from his friends, scientific and political colleagues, family and by Paul Langevin himself. Close to the Curies, Einstein, and Rutherford, this is a vivid account of a life lived at the core of physics turned upside down by the discovery of radioactivity and X-rays, by the theory of quanta, by relativity and by wave mechanics. Langevin himself was the originator of ultrasonics, a profound theorist of magnetism and of statistical mechanics and an evangelist for relativity. His utter commitment to rational science was equally applied to social justice. His life was deeply affected by both world wars and the political turmoil between them, during which he publicly opposed fascism and supported communist principles. In this personal, intimate account of his life, we find a generous, courageous, loving, brilliant human being. There is much in his understanding of humanity and the resolution of conflicts that remains relevant today.