Karl Kautsky was, for three decades before the First World War, the main authority on the intellectual heritage of Marx and Engels, the founding fathers of Marxism. His interpretation of Marx’s Capital and the basic laws and contradictions of capitalism was the standard reference point for both the foes and allies of Social Democracy. Jukka Gronow’s On the Formation of Marxism analyses Kautsky’s impact on the self-understanding of the European labour movement from his dispute over Revisionism with Eduard Bernstein to his polemics with V.I. Lenin over Imperialism and the Russian Revolution. Despite many political differences, Gronow shows that these authors shared a common understanding of the basic nature of capitalism, which in important respects differed from Marx’s critique of political economy.