This text offers an authoritative historiography of German socialist theorist Karl Kautsky and his impact on debates about the Russian Revolution and the contemporary left. Known as the “Pope of Marxism,” Douglas Greene examines the totality of Kautsky’s political career and dissects the fundamental opportunism and passive radicalism that defined his Marxism. He later examines the most substantive Marxist critics of Kautsky, namely Rosa Luxemburg, V. I. Lenin, and Leon Trotsky, while offering a critical assessment of the work produced by scholars and activists, Lars Lih, Eric Blanc, and Mike Mcnair, seeking to revive Kautsky. The New Reformism and the Revival of Karl Kautsky is an important addition to scholarship on the subject and a valuable resource for those interested in the Russian Revolution, German politics, socialism, Marxism, and contemporary left-wing debates.