Although the capability approach has had a tremendous impact on the development debate, it has had little to say about sustainable development. As several Human Development Reports have maintained, the last twenty years' gains in human development are not sustainable. The failure to include an integrate sustainability into the Human Development Index would thus give the wrong policy message. Drawing on the works of Amartya Sen and Thomas Scanlon, this article argues that sustainable development can be seen as a process of increasing legitimate freedoms, the freedoms that others cannot reasonably reject. Thus, Sen's vision of development as freedom is amended to suggest limits to freedoms. Forms of development which are not sustainable can be reasonably rejected due, at least, to the harm and blighting entailed. Based on this, it is argued that at country level of comparison the Human Development Index should be combined with the Ecological Footprint to reflect sustainability, and that the Human Development Reports should give way to Sustainable Development Reports.