This thesis examines the role of the EU’s foreign policy in the field of nuclear disarmament. The object of study is only limited to this realm, and not to other policy fields such as non-proliferation or combating nuclear terrorism. In assessing the EU’s ability to act as a global actor, this thesis proposes to answer, through the use of a qualitative empirical analysis, two research questions: (1) how and when does this nuclear disarmament policy work? and (2) why does this policy have more power (and success) in some cases while not with many others? First, this doctoral research examines how this policy works, by introducing its major features and tracing its key historical events. I find that this policy has its origins in the post-War period, but only became common, external, and coherent policy in the post-Maastricht treaty era during the preparations for the 1995 NPT Review Conference. Furthermore, this thesis explores the process which establishes that the EU acts internationally at multilateral fora and characterizes the role of EU institutions and member states in creating and implementing this policy. Here, I claim that the EU has a lack of consensus between member states on the different approaches and steps to achieving nuclear disarmament. The EU’s common and foreign policy works when member states decide to act in a coherent and consistent manner while producing effective policy outcomes. Second, this manuscript evaluates why this policy does not work in most cases. Thus, to evaluate its effectiveness and success, I use a small-N of empirical cases i.e., Iran, North Korea, India, Pakistan, and the Treaty on the Prohibition of Nuclear Weapons. I examine and compare the findings of each case through three analytical dimensions i.e., historical process-tracing, elements of the EU’s power (diplomatic, economic, normative and defence networks) and decision-making process (internal, external and outcome). I argue that the success and effectiveness of this policy is shaped by these factors, but I determine that the Union only produced an effective positive outcome in preventing Iran from acquiring nuclear weapons within the end goal of achieving global disarmament.