Throughout the twentieth century wide sections of the social democracy expressed their adhesion to reformism and to the faith in the peaceful evolution from capitalism to socialism, conceiving it not as the theoretical and practical antithesis of the capitalist system, but as a parliamentary movement playing a part inside that system. Noticing the advance of reformism inside the German Social Democratic Party, Rosa Luxemburg focused her efforts in the invalidation of the revisionist thesis, making use of a totalizing and dialectical conception of Marxism. Based on the analysis of the valuable critics addressed by Rosa to Bernstein’s thought, this article intends to delineate the main existent divergences between the two great currents of social democracy during the turn of the nineteenth to the twentieth century, as well as to point out the importance of the author's contribution to the Marxist theory.