Although the sustainability debate has already brought about considerable concep- tual progress, a pivotal dimension to sustainable development has so far been widely neglected. This article argues that, in addition to the ecological, economic and social dimension, sustainable development critically hinges on the moral dimension of institutional legitimacy. Institutional legitimacy is a moral dimension because it reflects the moral quality that people attribute to institutional arrangements needed to implement sustainability. Only if people can perceive these ‘rules of the game’ as fair, legitimate and mutually advantageous will they be willing to take them seriously, accept them and abide by them. As conventional mechanisms for creating and legitimising institutions are increasingly challenged, new governance mechanisms become increasingly important. Companies participate in these processes as corporate citizens: to the extent that they learn to carry ordo-responsibility for a fair and adequate institutional order, they contribute to empowering sustainability by corporate citizenship.