Introduction

At the moment of signing the early EU treaties, the founding member states were not aware of the long-term pervasive implications of EU membership. This pushed them to remove the constitutional obstacles or take special measures to ensure the efficient implementation of EU law at the national level. Over time they seem to have adjusted their constitutional strategy in the face of multi-level governance, following the rise of concerns regarding the legitimacy of that same law within national boundaries. In this introduction, we put forward a series of indicators and hypotheses aimed at investigating in a comparative way what we call the constitutional dynamics of multi-level governance, i.e. the dynamics resulting from the interaction among national constitutional actors when faced with the challenges and opportunities posed by EU law. We identify four main indicators (ratification of EU treaties, judicial review of legislation, parliamentary scrutiny in EU affairs, and the role of subnational entities) and formulate four research hypotheses in search of the contextual factors justifying the different constitutional approaches adopted by member states in regard to EU integration.