This study was commissioned by the European Parliament’s Policy Department for Citizens’ Rights and Constitutional Affairs at the request of the Committee on Constitutional Affairs of the European Parliament. It analyses the political and legal dynamics behind referendums on EU-related matters. It argues that we have entered a period of increasing political uncertainty with regard to the European project and that this new political configuration will both affect and be affected by the politics of EU-related referendums. Such referendums have long been a risky endeavour and this has been accentuated in the wake of the Great Recession with its negative ramifications for public opinion in the European Union. It is clear that referendums on EU matters are here to stay and will continue to be central to the EU’s future as they are deployed to determine the number of Member States within the EU, its geographical reach, its constitutional evolution and adherence to EU policies. Only now they have become an even riskier endeavour.