This PhD thesis examines how and why the EU developed its discourse about sustainable development over time, and in particular its ability to ‘speak with one voice’ as a supranational actor in international negotiations, attempting to shape but also being shaped by its involvement in multilateral diplomatic processes. More specifically, the thesis looks at how, over the period of 1992-2017, the EU has dealt with the economic dimensions of sustainable development, in particular globalization. Focusing on key summits (UNCED, WSSD, Rio+20 and the SDG process), it draws on and further develops a discursive institutionalist perspective to understand the interplay of discourses and institutions in the EU’s diplomacy and attempt to handle the challenges of a changing economic order and a (multifaceted) crisis of sustainability. In doing so, the analytical categories of the ‘content’, ‘conduct’ and ‘culture of discourse’ is developed and it is argued that only by studying the latter can we uncover the taboos in EU diplomacy that have a significant effect on outcomes. Moreover, the thesis is grounded in debates on ontology and epistemology and brings an array of methods and sources to bear on the empirical research. This includes 27 interviews, 2 x 6 months of participant observation, analysis of video material of key meetings, extensive archive work (ca. 2200 pages of documents from the Danish Ministry of Foreign Affairs’ archives) and discourse analysis. On the basis of the direct participation in the Rio+20 and SDG negotiations, the thesis offers an insider account. This positionality is used to explore how the EU works diplomatically.