Ever since the launch of the EU’s first military operations in 2003, the Council recognized the importance for the EU to learn lessons from all its missions. Indeed, preliminary evidence from primary and secondary sources suggests, that at least part of the CSDP developments of the past years in the field of military crisis management result from lessons as identified during the first CSDP military operations. But despite the EU’s objective to learn from its missions and tentative proof that learning is actually happening, CSDP remains characterized by a number of limits and drawbacks. But why? Why did the EU, in the field of military crisis management, learn some lessons from its missions, and not others? The thesis explores this empirical puzzle and qualitatively investigates this research question. In order to do so and from a theoretical perspective, it makes use of Organizational Learning Theory (OLT), which provides the theoretical and analytical tools to explain how learning and non-learning can possibly occur within CSDP. Empirically, it is based upon 85 interviews that were conducted across the institutional landscape of CSDP as well as with some UN and NATO staff. The first chapter lays out the theoretical, analytical and methodological framework of the thesis. The second chapter introduces its case studies – the EU’s first three military operations (EUFORs Concordia, Artemis, and Althea) and provides the necessary information to embed the research analyses to follow. The third chapter presents first empirical findings, focusing on the EU infrastructure for learning in EU military crisis management. It introduces the actors involved in the learning process and sheds light on the processes that account for learning in this field. Chapter four uncovers and explains the lessons identified from the case studies. Chapter five fleshes out the underlying causal mechanisms that explain why specific lessons were learned from the EU’s first three military operations. In order to do so, it contrasts one successful with one unsuccessful area of lesson learning. Building upon that, it adapts the model of learning processes within organizations as developed in chapter one. To support these theoretical and empirical findings, chapter six provides additional information on EU developments in all the remaining fields of lesson identification from chapter four. Concluding the thesis, chapter seven summarizes its empirical and theoretical findings, fleshes out its contribution, develops recommendations on how to improve the learning process in EU military crisis management and lays out research questions that emerge from its findings.