Sustainable development increasingly provides new norms in the international agenda for development assistance. As an international development actor the European Union (EU) integrates this notion into its objectives for development co-operation with African, Caribbean and Pacific (ACP) countries. This study, therefore, investigates how effectively the EU contributes to the sustainable development of Tonga, member of the Pacific ACP region. An operational mode of sustainable development should adopt an agenda that addresses the needs of the poor and adopts the objective to manage natural resources in a manner that allows economic growth and social development without irreversible impacts on the environment. In the bilateral development co-operation between the EU and Tonga the concept of sustainable development is now firmly established as an overarching objective. This research therefore investigates the correlation between Tonga’s agenda for sustainability and the development policy and co- operation the EU provides. In the policy framework that the EU adopts, addresses effectively many of the aspects of Tonga’s sustainable development. In the current framework of the tenth European Development Fund (EDF), in particular, the EU adopts appropriate strategies for the management of Tonga’s environment that support social and economic development. An analysis of the allocation of funds, however, shows that the promised policy strategies do not result in appropriate action. To contribute more successfully to the sustainable development of Tonga, the EU needs to integrate the development of the country into its own interests. The notion of cosmopolitan moral responsibility and distributive justice offers an incentive for the EU to do so.