This chapter provides a case-by-case assessment on how the various types of subnational authorities deal with the European Union (EU) but to provide a comparative analysis providing novel findings and interrelations that have not yet been explored by the abundant literature on this issue. It considers the case of the most powerful subnational governments, the so-called Regions with Legislative Powers (REGLEG), both in their respective national context and in aggregation: namely the seminal work of REGLEG during the European Convention and the praxis of their Brussels representation. The chapter describes the praxis of local governments in aggregation as to shape the national EU policy as well as by way of their engagement in their EU-wide associations. However, REGLEG problem was at a deeper level, and their common demands were tools for very different destinations: while some REGLEG wanted to use this advocacy to make EU decision-making truly multilevel.