The book is based on a complete translation into English of the 1987 biography, Langevin 1872-1946 Science et Vigilance by Bernadette Bensaude-Vincent.This new book includes some important revisions to the original text with numerous additional figures.The description of Langevin’s work in ultrasonics has been developed and expanded, including much new detail. French physicist, student of Pierre Curie, close friend of Marie Curie and Albert Einstein, Paul Langevin was one of the handful of influential European physicists who created modern physics at the beginning of the twentieth century.Scientifically, he is remembered particularly for his contributions to the atomic theory of magnetism, for the understanding of relativity and for his work on statistical kinetics.This establishes the context for his role as the ‘originator of modern ultrasonics’ during the First World War, and his influence on its growth as a scientific speciality during the subsequent two decades.Leading educationalist, he promoted education for women, and science as a core subject.He viewed science as a common good, to be shared for the benefit of all mankind, at a time of growing nationalism.The book focuses also on his political activism as a rationalist and humanist, including his public anti-fascist stand and communist sympathies leading to his house arrest during the Second World War.This portrait of a committed scientist regains a special relevance in the 21st century.