Though primarily associated with the most prominent figures in the history of European Marxism—Lenin, Luxemburg, Hilferding, and Bukharin—the theory of imperialism was actually developed through lively and engaged debates within the Second International from 1898–1916. This volume assembles and translates for the first time the main documents from this debate, and features contributions from Karl Kautsky, Parvus, Otto Bauer, Karl Radek, Anton Pannekoek, and Trotsky, among numerous others.