NATO’s role as the defence alliance in the Euro–Atlantic space is uncontested, but with the emergence of other security organisations in Europe such as the European Union (EU) and the Organization for Security and Cooperation in Europe (OSCE), it is certainly no longer the only security actor. This chapter argues that the EU is NATO’s most important partner for maintaining peace, security, and stability in the Euro–Atlantic community thanks to their membership overlap, established cooperation frameworks, unique toolboxes, and comparative advantages. Looking back at over 30 years of cooperation, this chapter outlines the evolution of their relationship, depicts major milestones, and highlights key challenges that need yet to be overcome. It elaborates on both organisations’ most recent initiatives and projects, including their new strategic doctrines and the debate on European strategic autonomy, and examines how these contribute to enhanced cooperation and shared responsibility between NATO and the EU.