The year 2017 has been an uneasy one for the EU, with so-called Brexit on the horizon and the rise of populist euroskepticism in a number of Member States. This year, with the tenth anniversary of the Romanian and Bulgarian accession to the Union, is a good year to pause and reflect over the life and future of the Union. In this work, we envision the next decade with Europe 2020 strategy and review the fruits of the 2004 accession in Central and Eastern Europe. What has the Union achieved? Which policy areas are likely to change and how? How successful, and by what measure, has the accession of the 10 Member States in 2004 been? Reviewing European Union Accession addresses a wide range of issues, deliberately without any thematic constraints, in order to explore EU enlargement from a variety of perspectives, both scientific and geographical, internal and external. In contrast to the major works in this field, we highlight the interrelated, and often unexpected, nature of the integration process – hence the subtitle, unexpected results, spillover effects and externalities.