This study examines the security governance between NATO and the EU. I specifically focus on whether the EU-NATO strategic partnership has led to the institutionalization of complementarity between the two institutions. I empirically scrutinize the NATO-EU strategic partnership in the field of security governance. Then, I question whether Permanent Structured Cooperation (PESCO) and the European Intervention Initiative (E2I) have marked a drift away from complementarity. PESCO and the E2I have mostly been considered as a challenge to the U.S.-led Atlantic Alliance. On the contrary, I see such initiatives as an enhancer for implementing better practices of security governance. In the first part of the study, the security governance approach to NATO-EU inter-institutional relations is explained. In the second part, the NATO-EU Strategic Partnership and initiatives for stronger European military capabilities are explored. In the last part of the study, the durability of the NATO-EU strategic partnership is questioned. I come to the conclusion that despite the different approaches to Euro-Atlantic security among the leading actors, such as the U.S., France and Germany, the NATO-EU Strategic Partnership will likely continue with complementarity in security governance between these two institutions.