The European Union (EU) has long strived to act collectively in the face of international challenges such as poverty, hunger and state fragility beyond its borders. While the EU member states and institutions seek coherent responses to these challenges, they also have partly competing agendas. Yet there has been increasing agreement on collective action. To understand this agreement, this thesis asks how policy professionals contribute to the advocacy of policy norms for collective action between the EU institutions and the member states. The research analyses policy processes in EU development cooperation since the early 2000s. In development cooperation the EU’s effectiveness has been particularly contested because of the combination of competing ideas about the EU’s role and about how to achieve effective and sustainable development. The research finds that, while formal decisions about collective action remain in the hands of member states, transnational networks of policy professionals in the EU institutions, member state bureaucracies and civil society contribute to shaping the terms of debate regarding the EU’s role in effective development cooperation. These network interactions, which form around institutional decision-making centres, transcend the organisational boundaries of member state bureaucracies, EU institutions and civil society organisations. These findings fill a gap in our understanding of how EU norms governing collective external action are advocated as existing research has tended to focus on how institutional structure facilitate state coordination. By concentrating on the cases of Germany and the United Kingdom and their engagement with the EU institutions, the research revises existing, dominant views on norm advocacy in EU external action: It links the previously little related concepts of norm advocacy and discursive networks to analyse the agency and scope of policy professionals in the advocacy of EU policy norms; and it provides new empirical insights into the role of these policy professionals for collective action between the EU institutions and the member states in development cooperation.