What explains France’s unique Left? Many works have reflected upon the importance of Marxism in France, yet few studies have been devoted to the man who did most to introduce Marxism into its political culture: the today near-forgotten figure of Jules Guesde. It was with Guesde that Karl Marx drafted the world’s first Marxist program, and Guesde who aroused the enthusiasm of countless worker-militants who saw him as their most important leader. Jules Guesde represents the first book-length study of the French socialist leader translated into the English language. For the radical Left today, Guesde is often considered a dogmatist who supported the Union sacrée during World War I and rejected the Bolshevik revolution; for the governmental Left, he embodies an intransigent ideologue who held back the modernization of the French Left. Throughout Jules Guesde, Jean-Numa Ducange argues that it is impossible to study the history of the French socialist movement without a close look at this singular figure and offers a fuller picture of the deep transformations of the Left and Marxism in France from the late 19th century up to the present.