INTRODUCTION

Having evolved from a series of sectoral integration agreements between six European countries into a truly autonomous legal order comprising 27 Member States and their nationals, the European Union epitomises the idea of multilevel polity building through law. Its fundamental premise, expressed in Article 2 of the Treaty on European Union (TEU), is that each Member State shares with all the others a set of common values. One of these values is the rule of law. Concisely understood as the idea of constraining public powers by norms beyond their control, it assembles a number of formal and substantive requirements ensuring fairness and accountability.

A meaningful definition of the rule of law in the EU legal order needs to capture its particular constitutional identity and reflect the intertwinement of national and Union levels of power. Indeed, the Member States have not only founded the EU and relinquished sovereign rights to the latter, but they also have their say in its policy- and rule-making and, for the most part, implement and enforce the rules thus made. Based on EU law and mutual trust, they cooperate with each other in a wide range of administrative and judicial matters. While collectively involved in a variety of formal or informal consultations, accords and agreements on issues of EU relevance, the Member States may, finally, defend their interests4 in litigation before the EU Courts on the application, interpretation or validity of EU law, as well as each other's respect for (the rule of) EU law.