Abstract: This thesis sets out to investigate the role of the European Union in international climate change politics over a period of two decades, more specifically from 1990 to the end of 2008. Studying the EU in relation to climate change is considered highly relevant, as it is a policy issue of overall importance in today’s world politics. Further to this, the European Union is considered to have been and still is an important and pivotal player in international climate change politics. Thus investigating the role played by the European Union in this sphere is certainly relevant. The focus in this thesis is the role of the EU seen from the perspective of leadership, as this is a denomination often connected to the EU, when speaking of climate change. A long-term perspective in the analysis is applied, in order to fully capture and understand the EU’s engagement over time. The overall aim is to bring forward an explanation as to why EU leadership has been somewhat inconsistent and fluctuating in the period under investigation, when it from the outset seems to have had a clear ambition of leadership in this sphere. The guiding question is sought answered by assessing the EU’s engagement in international climate change politics through the glasses of a role-theoretical framework, emphasising the analytical variables of role-conception and role-performance. Through a theoretical understanding of leadership, the two aspects of role are held against this background in order to measure the degree of leadership projected and exerted. Through the analytical distinction between role-conception and role-performance, several important and interesting findings are generated, which provides a basis for understanding and explaining EU fluctuating leadership. The main findings of the two analytical chapters are applied and held against each other identifying a conception-performance gap at points in the period analysed. Taking a closer look at the variables surrounding this gap a possible understanding and explanation of EU fluctuating behaviour emerges. Three main explanatory variables are identified and assessed through the use of the underlying perspectives on role as represented in the role-theoretical framework and overall assist to put forward an overall explanation and understanding of EU fluctuating leadership as a result of a range of internal and external determinants.