The monograph examines the main stages of the political, literary and journalistic activities of Yu. O. Martov (Zederbaum), a leader and theorist of the Mensheviks, a staunch and consistent supporter of the idea of ​ ​ democratic socialism. The many years of political dialogue between Martov and Lenin, the clashes between Martov and Stalin, the participation of the leader of Menshevism in the Russian revolutions and the international socialist movement are just some of the most striking episodes of this interesting book, in which many new archival materials were used for the first time.