Treaties between EU Member States as quasi-Instruments of EU law

This chapter deals with legal instruments that formally speaking are not EU legal acts, but whose function is so intimately linked to the EU legal order that they can be called ‘quasi-instruments of EU law’. These are treaties concluded between all of the EU Member States (complementary agreements) or between only a selected number of them (partial agreements), in close connection with the operation of the European Union. Such international ‘side agreements’ have lately become rather prominent and controversial, particularly in the context of the euro crisis. The chapter discusses the variety of reasons for the adoption of those instruments, as well as the questions of legitimacy and compatibility with EU law which they raise.