This chapter provides an overview of the European Union and its member states. It first explains why the member states matter in the EU before discussing the role of member states in the EU, with particular emphasis on three approaches to understanding member state-EU relations: intergovernmentalism, institutionalism, and governance approaches. It then examines the Europeanization of the member states as well as the domestic politics approach, which claims that it is impossible to understand the EU without considering domestic politics. It concludes by presenting the logic and structure of this volume: how the relationship between the EU and its member states will be portrayed in the chapters that follow.