In his parting word about Marx at Highgate Cemetery, Engels characterized his friend as 'before all else a revolutionist.' This was a true summation of Marx both as a man of action and as a thinker. For as a theorist Marx was before all else a theorist of revolution. The revolutionary idea was the keystone of his theoretical structure. Marxism, as he fashioned it with the assistance of Engels, was in its essence a theory and program of revolution.