EUNAVFOR Operation Atalanta has been the first maritime operation of the European Union and it has certainly been successful given the significant decrease of pirate attacks off the Somali coast. However, various issues have been raised concerning its legal basis under international law and its legal framework, including questions of responsibility. These issues are particularly interesting since the EU has a more integrated legal order than other organizations involved in such operations (eg UN, NATO). The present article attempts to address these issues against the background of international and European law. Even though the legal basis of the Operation is clear from a European law perspective, there have been certain misconceptions concerning the legal basis of the Operation under international law. The delineation of the Operation's legal framework requires a careful analysis of the rules applicable to each of its phases and of its addressees, since each phase is subject to different rules which are binding on different actors. Finally, there is an extensive discussion of questions of responsibility, which were heavily influenced by the applicable Rules of Engagement and of the actual conduct of the Operation. The conclusion is that, at least on the high seas, responsibility should primarily rest with the flag States rather than with the EU. However, in most cases the EU is indirectly responsible for violations of international law, except in cases where suspected pirates are transferred to third States pursuant to EU agreements with such States, in which case it bears primarily responsibility.