Brexit, a major change in EU politics and a diminution of the EU population, economic weight, and prestige, has attracted relatively little European attention, in contrast to the scrutiny it has understandably received in the United Kingdom. This chapter introduces arguments about why the impact of Brexit on the EU will be substantial, setting the stage for contributors to this volume to examine its consequences for the Single Market and economic governance, for the legal order and social construction of the EU, and for the future external orientation and institutional forms of the EU. This chapter also outlines how Brexit will provoke a more fundamental disruption in the ideological climate of the EU, as the most reliably ‘liberal champion’ in the EU departs. Among the themes that emerge across this volume, this chapter notes discussions about whether Brexit will leave the EU with greater integrative potential; whether British intellectual influence in Europe will continue; and whether the EU itself will learn lessons about the viability of austerity and fiscal governance as it seeks to respond to its own challenges of legitimacy.