This book, first published in 1962, is a collection of essays on the ideological origins of the European revolutionary movement. The first essay in the collection is devoted to Saint-Simon who, though not a revolutionary in the ordinary sense, was the begetter of the many ideas which became stock-in-trade of the nineteenth century revolutionaries. The essays that follow are on Marx and the Communist Manifesto, Proudhon, Herzen, Lassalle and Sorel; on the foundation and early history of the Russian Communist Party; on the histories of the British and German Communist Parties; and on Lenin and Stalin.