This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book provides a comprehensive view of their conceptual formulation, institutional recognition and application in practice within the current institutional and legal settings of the European Union (EU). It describes the period from the Laeken Declaration of December 2001 to the UK EU referendum of June 2016 and its immediate aftermath. The book draws on a comprehensive map of the praxis of Multilevel Governance in the EU through the eyes of those institutional actors that are not privileged by EU law in terms of their institutional recognition in the Treaties and in the formal decision-making process. The acceleration of European integration, first with the Single European Act and soon after with the Maastricht Treaty, led to a progressive transfer of competences to the EU level. The empirical observation of actor-centred EU institutional dynamics sometimes provides an unedifying picture.