This study develops a close-textual and comparative analysis of two influential doctrines of "proletarian revolution" that appeared during the Second International period (1889-1914): "German orthodox Marxism" and "French revolutionary syndicalism." It considers the effect of the dramatic mass-organizational development of the European workers' movement upon both doctrines - in particular, how this shaped the key concepts of "scientific socialism" and "direct action." In this light, the first chapter of the study examines the relationship of both doctrines to "classical" Marxist and anarchist ideology. The main body of the study then outlines the historical development of both tendencies - their "formation" and "decomposition." This analysis challenges prevailing historiographical interpretations of several significant revolutionary thinkers: on the Marxist side, Friedrich Engels, Karl Kautsky, and Rosa Luxemburg; on the syndicalist side, Fernand Pelloutier, Émile Pouget, and Georges Sorel. It also reframes the political history of two major pre-First World War institutions: the Sozialdemokratische Partei Deutschlands and the Confédération générale du travail.