The chapter first explains what Europeanization means and outlines the main approaches to studying this phenomenon. The second section describes why this concept has become so prominent in research on the European Union (EU) and its member states. In the third section, the chapter reviews the state of the art with particular reference to how the EU affects states (‘top-down’ Europeanization). It illustrates the theoretical arguments with empirical examples. Similarly, the fourth section examines how states can influence the EU (‘bottom-up’ Europeanization) and provides some theoretical explanations for the empirical patterns observed. This is followed by a section that presents an overview of research that looks at linkages between bottom-up and top-down Europeanization, and considers the future of Europeanization research with regard to the EU’s recent and current crises and challenges. This chapter argues that Europeanization will remain an important field of EU research for the foreseeable future.