The thesis defends a particular version of cosmopolitanism: statist cosmopolitanism. Its point of departure is the current debate in international political theory on the moral standing of boundaries and the scope of distributive equality. Against existing cosmopolitan approaches, it is argued that states have an intrinsic normative standing and constitute the most relevant agents of global justice. Against non-cosmopolitan approaches, it is argued that the defence of compatriot favouritism in the distribution of egalitarian obligations rests on a confusion between motivation and principles in ethical reasoning. More positively, this research claims that the state is not only compatible with the cosmopolitan defence of distributive equality but also necessary for its realization. The work is divided in three parts. The first part introduces some preliminary observations and illustrates the historical roots of the controversy between statism and cosmopolitanism with particular reference to the Enlightenment. The second part criticizes cosmopolitan and non-cosmopolitan approaches to global justice and defends both cosmopolitanism and the state. It illustrates how the limits of cosmopolitan and non-cosmopolitan arguments are due to more fundamental methodological flaws, regarding the place of ideal and non-ideal considerations in normative theories of global justice. The third part reconciles the defence of cosmopolitanism with the defence of the state both at the level of principle and at the level of agency. At the level of principle, it shows how cosmopolitan egalitarian obligations to relieve relative deprivation may be grounded on sufficientarian responsibilities to relieve absolute deprivation. At the level of agency, it argues that those obligations ought to be understood politically - not just morally - and that states constitute the most relevant agents for their realization. Finally, the research clarifies how the transition from principles to agency could be made by local cosmopolitan 'avant-gardes', responsible for acting within the state and thinking beyond it.