The role of national parliaments in EU matters has become an important subject in the debate over the democratic legitimacy of European Union (EU) decision-making. Strengthening parliamentary scrutiny and participation rights both at the domestic and at the European level is often seen as an effective measure to address the perceived 'democratic deficit' of the EUthe reason for affording them a prominent place in the newly introduced 'Provisions on Democratic Principles' of the Union (in particular Art.12 TEU). Whether this aim can be met, however, depends crucially on the degree and the manner in which national parliaments actually make use of their institutional rights. This volume therefore aims at providing a comprehensive overview of the activities of national parliaments in the post-Lisbon era. This includes the 'classic' scrutiny of EU legislation, but also parliamentary involvement in EU foreign policy, the use of new parliamentary participations rights of the Lisbon Treaty (Early Warning System), their role regarding the EU's response to the eurozone crisis and the, so far under-researched, role of parliamentary administrators in scrutiny processes. In this introduction, we provide the guiding theoretical framework for the contributions. Based on neo-institutionalist approaches, we discuss institutional capacities and political motivation as the two key explanatory factors in the analysis of parliamentary involvement in EU affairs.Keywords: National Parliaments, EU, Scrutiny, Lisbon Treaty, Neo-Institutionalism, democratic legitimacy 'Maybe not formally speaking, but at least politically speaking, all national parliaments have become, in a way, European institutions' (Van Rompuy 2012). Whether it is indeed true -or 2 even desirable -that national parliaments have become 'European institutions' in a more narrow sense, is open to debate (Cygan 2013: 21). Yet the statement certainly suggests that the former 'losers' of the European integration process have come a long way. For a long time the role of national parliaments was not formally recognised at the European level, and in the domestic arena the integration process provided ample opportunities for the executive actors to bypass legislatures and strengthen their hold on policy-making. Concerns about a growing democratic deficit were addressed through repeated and substantial expansions of the powers of the European Parliament, whereas national parliaments remained on the margins.However, over time the 'victims' of integration have learned 'to fight back' and obtained new opportunities for participation in domestic European policy-making. It was a slow and uneven process through which in they improved their institutional position, but they gained increased rights to scrutinise European affairs and to control the way in which ministers and officials represented national interests in Brussels. Today, the Lisbon Treaty not only mentions the role of national parliaments explicitly (article 12 TEU), but it also gives them a role within the EU's legis...