Theorien über den Mehrwert
From 1885 to 1890 he spent time in London, where he became a close friend of Friedrich Engels. His position as a prominent Marxist theorist was assured in 1888, when Engels put him to the task of editing Marx's three-volume work "Theories of Surplus Value". In 1891 he co-authored the Erfurt Program of the Social Democratic Party of Germany (SPD) together with August Bebel and Eduard Bernstein.