In today's interconnected, differentiated and polarized European Union, sub-regional groupings between the national and EU level are a highly relevant empirical phenomenon. Despite their importance, the interactions between sub-regionalisms and EU actors and processes remain under-researched. This book fills this large research gap by applying a systematic comparative approach. Taking into account a huge corpus of empirical data gained through expert interviews and document analysis, this study provides the conceptual and empirical groundwork necessary to better understand EU sub-regionalism. For the timeframe between 2009 and 2018, it analyses the scope of sub-regional cooperation in the Baltic, Benelux and Visegrád groupings. Moreover, it compares their respective institutional design, political identity, foreign policy as well as their external recognition by EU actors. A special focus lies on their role in six EU policy cases, including EU budget negotiations, TTIP, the drafting process of the EU Global Strategy, refugee relocation quotas, the revision of the posted workers directive and defence cooperation through PESCO. In its conclusion, this book identifies two sub-regional archetypes: functional hubs and political tools. While differing in many regards, the shared assessment finds that the overall bridge-building efforts of the Baltic, Benelux and Visegrád sub-regionalisms exceed their spoiling potential.