In recent years, cooperation between the UN and the EU in the realm of international peacekeeping has gone through major changes, including the remarkable achievement of a ‘Joint Declaration on UN-EU Cooperation in Crisis Management’ through which both organisations promised their primary role and responsibility for the maintenance of international peace and security. Although the EU has reiterated its commitment to play a decisive role as a reliable peacekeeping actor within the UN framework, the decision of the EU to not always engage in international peacekeeping operations alongside the UN is puzzling. The aim of this dissertation is to examine the major driving forces behind decision-making which may determine the likelihood of EU cooperation with the UN in international peacekeeping, by asking: ‘under what condition do EU Member States lead UN-related peacekeeping operations?’ Using a revised two-level game approach, this thesis identifies the most important chief negotiators involved in negotiations, and analyses the dynamics of decision-making between the UN and the EU on the issue of international peacekeeping at two different levels: international UN level and domestic EU level. Variables and conditions under which chief negotiator(s) are more likely to provide active leadership to drive the EU to decide to engage in a peacekeeping operation are investigated with insights from two prominent IR theories; realist and social constructivist theories. Hypotheses drawn from each theory and the roles of chief negotiators are examined in each of three cases selected: Operation Artemis (2003), EUFOR RD Congo (2006), and EUFOR Tchad/RCA (2007). With the key research outcomes from a comparative analysis of the three case studies, the thesis aims to contribute to comprehensive debates on the role of the EU as a promising partner of the UN in international peacekeeping.