Cross-border threats have increased the importance of internal and external security for the EU Member States. In this respect, the 2020 EU Security Union Strategy provides a coherent regulatory framework for numerous concepts, strategies and action plans in the field of EU security measures concerning, e.g. Critical infrastructure and network information systems Counter-terrorism Institutional intelligence cooperation These and numerous other security issues are discussed in the book, providing clarification and insights into the extent and limits of European security cooperation and the degree to which Member States remain solely responsible for their own national security, especially in light of recent ECJ case law. The eitors Jan-Hendrik Dietrich is professor at the Federal University of Applied Administrative Sciences, Germany; Arne Pilniok is professor at the University of Bielefeld. The authors are distinguished experts from academia and stakeholder institutions.