International actors have increasingly recognized, and acted upon, the idea that security and development go hand in hand to foster democratization and human security in postconflict settings. One such practice of merging security and development issues, security sector reform (SSR), in which the goal is to reform national security institutions in order to make them more accountable, democratic and responsive to citizens' needs, has been championed since the 1990s by a variety of actors as a key element of the democratization and peace/statebuilding toolbox. Yet, despite both conceptual and practical developments, there is no emerging consensus on the how SSR impacts postconflict peacebuilding and democratization across a variety of countries and sectors and what shapes the determinants of postconflict SSR activities. This paper seeks to provide for such an explanation of the variation in impacts and consequences of SSR. It does so by addressing the issue of SSR in a comparative perspective, with the European Union (EU) as the central part of the analysis. The main research question is : Why, how, when and where is the EU able (or not) to bring about institutional change through its whole security sector reform machinery?. The main argument of this paper is that the ways in which security sector reform activities impact democratization and peacebuilding processes depends on a set of background knowledge, routines and frames that inform how actors think from when designing and implementing SSR projects. The theoretical argument is thus linked and builds on two different research fields: it follows the recent development of the practice turn in international relations by looking, using organizational theory concepts of routines and frames, at what makes certain practices possible and it operationalize, using the logic of practice, the grand and abstract notion of liberal peace developed within critical peacebuilding research circles. In short, this paper provides a theoreticallyinformed framework to understand the impacts of SSR on democratization and peacebuilding in postconflict settings. To do so, it provides a detailed and comprehensive empirical study of one key actor, the European Union, across three countries (DRC, Bosnia and Albania) and focusing on the civilian side, of SSR - rule of law, police and justice sectors , where the EU is most active. The empirical part takes as its point of departure the general idea that the main objective of SSR is to bring needed, sustainable institutional change into local security institutions. Institutional change in the security sector reform refers to changes that affect the structure (the form of the security sector, organizational and administrative) and the function (the content of the activities undertaken by the security sector). In the end, this paper contributes to the advancement of knowledge by adding a theoretical lens on peacebuilding that builds on the practice turn in international relations and organization theory, by providing detailed case studies of institutional change within police and justice reforms in different contexts and by highlighting mechanisms of success and failure of peacebuilding that could be useful to policymakers. [ABSTRACT FROM AUTHOR]