Introduction Long criticized for being unable to deal with conflicts in its neighbourhood, since the end of the 1990s, the European Union (EU) has concentrated its efforts in building an effective military crisis management capability through the institutionalization of its Common Security and Defence Policy (CSDP). A lot of attention has been paid to the origins and the formal institutionalization of the CSDP (see also Chapter 4), while the performance and day-to-day development of this policy remains a less studied subject. Without neglecting the role of the member states and external events in explaining the origins of the CSDP, this chapter looks at how institutions have shaped the design and performance of military crisis management operations. In line with previous chapters in this section, this chapter highlights the significance of institutions in EU crisis management, but it does so from a different theoretical perspective, that of sociological institutionalism. This chapter argues that sociological institutionalism can be helpful in illuminating some of the puzzles surrounding this policy and, in particular, the design of crisis management institutions and missions, the performance of institutions, and its impact on the policy substance and implementation. In support of this claim, the chapter looks at different factors, building on the insights of sociological institutionalism and organization theory, inter alia, institutional isomorphism, organizational routines and socialization processes.