This chapter aims at distilling 20 years of research in intercultural and social justice education resulting in systematically delineating the notion of global citizenship education (GCE) as a new educational perspective providing an innovative angle to reframe old dilemmas of citizenship in diverse societies. GCE provides a value-based idea of citizenship as ecological belonging to the world. In this chapter, the author argues that while the idea of global citizenship does not have legal value, a new environmental ethic is rooted in it, by closing the traditional gap between citizenship and sustainability, human rights and environment, and social sphere and biosphere.