The years between 1864 and 1867 were among the most fulfilling of Marx’s life. Not only were these the years in which he wrote up Capital, it was also the period in which he became an active and influential participant in the International Workingmen’s Association, founded in London in 1864. Almost by chance, it fell to Marx to compose the inaugural address of the Association and formulate its rules. In this lecture, Gareth Stedman Jones argues that in writing the address, Marx made his greatest and most permanent contribution to the International: he had formulated the new social democratic language of the 1860s, both in the definition of the political and social end of the association, and in a global diagnosis of the worker’s condition.