Countering hybrid threats has become an important area of focus in the updated security strategies of the European Union (EU) and NATO, as both organizations and their largely overlapping Member States increasingly confront ‘sub-threshold’—below the North Atlantic Treaty’s Article 5—and ‘grey zone’ regional and global security challenges. In 2016, when both organizations agreed on the initial catalog of topics for cooperation, countering hybrid threats was identified as an emerging theme in cooperative policy-making and institutional adaptation. Prior to the alarm caused by the 2014 annexation of Crimea, the subsequent invasion of Eastern Ukraine, and the establishment of a military presence in Syria by the Russian Federation, these events were likely the main triggers for the intensification of such inter-organizational cooperation. In July 2016, the first Joint Declaration between the EU and NATO was signed in Warsaw, outlining seven areas for enhanced cooperation, including countering hybrid threats. In December 2017 and 2018, the EU and NATO Councils endorsed 74 specific actions, with 20 aimed at combating hybrid threats. Since then, both organizations have compiled annual progress reports and assessments of their cooperation. This chapter focuses on the main achievements and underlying theoretical considerations of this politically and operationally complex multi-domain security cooperation, which is still searching for a sustainable program and effective implementation strategy that, if successful, could mutually combine the organizational strengths and meet the security needs of its members in a rapidly changing security environment.