Hybrid threats posed by various combinations of state and non-state actors have presented considerable transnational challenges to EU-members and NATO-allies. This ongoing rise of hybrid threats, ranging from political instability in Eastern Europe and the Middle East to the resulting mass refugee influx and terrorism in the European neighborhood, stress the need to timely discuss important questions about hybrid threats and the venues for effective Euro-Atlantic cooperation, including post-Brexit policy implications. This edited volume presents comprehensive analyses from various experts on these interrelated issues, and, thus, represents an essential source for scholars and practitioners of European politics and international relations with an interest in contemporary transnational security issues. In addition, this book will be useful as up-to-date coursework material for courses on European security and foreign policy, international security and strategic studies, unconventional warfare, and transatlantic relations.