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This paper analyzes the process of legal formalization of the European Union's commitment to sustainable development, both as an internal and external policy objective, which was completed with Lisbon Treaty. Following the entry into force of this Treaty in 2009, sustainable development features prominently in the new Article 3 of the Treaty on European Union, the key treaty provision laying down the basic objectives of the Union. The new references to sustainable development in the Treaty are the culmination of a gradual process of emergence of sustainable development principles in the political and legal discourse of the EU, which started in the late 1980s and accelerated in the wake of the Rio Conference in 1992. The paper provides a chronological overview of this process and its main steps, with a special focus on the EU's Sustainable Development Strategy first launched by the European Council in 2001, and the Declaration on the Guiding Principles for Sustainable Development adopted by the European Council in 2005. It addresses the complex interactions between the three dimensions of sustainable development (economic, social and environmental) and the tensions between the Sustainable Development Strategy and the EU's parallel Lisbon Strategy for Growth and Jobs. The concluding part of the paper explores the potential legal implications of sustainable development as an objective and as a principle in the context of EU constitutional law. The ambiguity of the concept and the lack of relevant case-law thus far make it difficult to draw firm conclusions as to what the actual impact of the constitutional recognition of sustainable development as a Union objective might be. It is argued that, in view of its inherent ambiguity, sustainable development might be invoked not only to justify a stronger integration of environmental protection objectives in other policies but also, paradoxically, to temper the ambition level of environmental policies in the interest of economic growth.