Abstract This article provides a theoretically informed analysis to understand why and how security sector reform (SSR) norms have been integrated into the European Union (EU) security architecture. It investigates the role of epistemic communities in advancing the security-development/good governance agenda underpinning SSR and in fostering institutional learning. Shared knowledge and expertise have driven policy and institutional change, since they yielded a new policy consensus that transformed the EU's approach to security in the post-cold war international system. Empirical findings, based on content analysis and on 25+ semi-structured interviews, suggest that transnational expertise-based networks did play a leading role in shaping SSR conceptual development. However, they also show that cleavages between epistemic communities across the security/development dividing line are ultimately responsible for the EU's failure to achieve a coherent SSR policy implementation. Moreover, the article concludes ...