In his philosophical writings, Marx develops a conception of self-realization which includes a conception of positive liberty. Based on his critique of deontological ethics and law he rejects the idea that negative liberty is sufficient to realize emancipation and to overcome alienation. In the central concepts of Marx's philosophical anthropology (alienation, recognition, species being), a conception of positive liberty is integrated which will be made explicit here. In the third part of my chapter, it is shown that in his program of a critique of political economy Marx also uses a conception of positive liberty as a guiding principle. In the fourth part, the way in which Marx's conception of positive liberty fits into a philosophical tradition that can be labeled as "post-kantian perfectionism" is discussed. In the final fifth part, two fundamental problems in Marx's conception are considered and it will be shown why and in which sense the conception of positive liberty identifiable in Marx is systematically still important (if some philosophical corrections need to be made).