In 2008, the Council of the European Union adopted a 'Comprehensive Approach' that outlines a strategy for securing gender mainstreaming; two years later, the Council introduced a set of indicators to assess its implementation. The EU was responding to the United Nations Security Council's call for regional institutions to assist in implementing Security Council Resolution (UNSCR) 1325, adopted on 31 October 2000, concerning women, peace and security. This resolution sought to meet the 'urgent need to mainstream a gender perspective into peacekeeping operations'. Considering that prior exposure to gender issues, resources and well-established relations with civil society and gender advocates are lacking, adoption of both the Comprehensive Approach and the indicators, as well as the structures and procedures established since then as part of the EU's Common Security and Defence Policy, requires some explanation. This article draws on feminist institutionalist approaches to argue that the impetus for change came from individuals and groups within the EU who were involved in external networks, both above and below the supranational level, who seized on institutional idiosyncrasies that, in contrast, also shaped implementation of UNSCR 1325 in important ways. institutional hurdles that have impeded the integration of gender into the CSDP.Overall, 'gender mainstreaming is a demanding strategy, which requires policy-makers to adopt new perspectives, acquire new expertise and change their established operating procedures' (Pollack and Hafner-Burton 2000, 450; see also Weiner and MacRae 2014, 6).As noted in the European Commission's 1996 communication 'Incorporating equal opportunities for women and men into all community policies and activities' (when this strategy was first introduced into the EU), gender mainstreaming requires a Comprehensive Approach that involves 'mobilizing all general policies and measures specifically for the purpose of achieving equality by actively and openly taking into account … their possible effects on the respective situations of men and women (gender perspective)' (EC 1996, 2).Gender mainstreaming affords 'a new modus operandi for its realization' (Weiner and MacRae 2014, 5), because it tries not only 'to change or tackle structural inequalities, [but] aims at changing institutionalized inequalities' (Paantjens 2005, 335, emphasis in original).This makes implementation of UNSCR 1325 a particularly challenging task in the field of security and defence and explains, in part, the critique of scholars such as Lombardo and Meier (2006, 151), who observed that, even ten years after the Commission's communication, 'gender mainstreaming has not been effectively implemented in the EU', and that EU documents were 'gender-blind ' (ibid., 158). Because much remains to be done, it seems important to examine how gender mainstreaming is being implemented in the CSDP and why it has taken hold, because defining the driving forces thus far should provide insights into what might still be needed.Gender...