The theory of imperialism is usually associated with some of the ‘big names’ in the history of European Marxism, such as Lenin, Rosa Luxemburg, Rudolf Hilferding and Nikolai Bukharin, alongside whom the English Progressive John Hobson is usually mentioned. However, little is known about the development of Marxist theory on this subject besides the books of these figures. This volume assembles for the first time the main documents of the international debate on imperialism that took place in the Second International during the period 1898–1916. It assesses the contributions of the individual participants to the developing theory of imperialism, placing them in the context of contemporary political debates.