In the late 1930s, Langevin often claimed that dialectical materialismDialectical materialism allowed him to better understand his own discipline. This chapter discusses this claim through a close examination of what he meant by dialectical materialism. Langevin was initiated to dialectical materialism through the Cercle de la Russie neuve and his family environment. Langevin was Marxist in the sense that he adopted a specific view of dialectical materialism as the heritage of the French Enlightenment and the triumph of rationalism. While he contributed to spreading this Marxisme à la française, Langevin nevertheless remained convinced that scientific ideas were the driving force of history.