The financing of Common Security and Defence Policy (CSDP) operations questions the capacity of the European Union (EU) to contribute to crisis management. Instead of a single mechanism that would be used for all kinds of missions, different mechanisms are applied, depending on the objective and nature of the operation. The aim of this article is to explain change and inertia in the way these operations are funded. The first section is dedicated to the presentation of the legal framework and its characterization as a fragmented and flexible structure. It allows for identifying the changes that have affected the financing of EU operational activities abroad. The main problems and challenges in financing CSDP operations are dealt with in the second section. The third section explains: (1) why changes have occurred in some areas and (2) why inertia has prevailed in other areas in spite of the problems previously identified. Three main explanations are tested. Rules and practices in financing CSDP operations (1) result from operational needs, (2) are influenced by proactive supranational institutions and (3) are triggered by the Member States. These three explanations are not mutually exclusive. They all contribute to the understanding of CSDP operations, but the relative importance of each factor depends on the operations at hand. Generally, the second factor is at play in the case of civilian operations, while the third one prevails with regard to military operations. The first factor brings a complementary explanation to both kinds of operations.