What does it mean to be a European citizen? From the treatise, the question is easily answered: Being a European citizen means being a citizen of a Member State of the European Union (EU). Outside the treatise, however, the question is at the core of a significant (r)evolution. From the last years, the European Parliament, the European Commission, and the European Court of Justice are all newly and increasingly paying attention to the very substance of the legal link between the EU and its citizens. From the last couple of years, the legal doctrine of the ‘genuine link’ – which was previously regarded as a ‘romantic’ (outdated) notion – is now becoming central to evaluate the legitimacy of the Member States policies regarding nationality acquisition and loss. This paper aims to investigate the content of the genuine link doctrine, from ancient international law to contemporary EU law. It claims that, in special circumstances related to naturalisation, it is possible to come to a clear definition of the genuine link between the citizens and the state based on habitual residence. Finally, the paper proposes to consolidate the penetration of the genuine link doctrine in EU citizenship law by offering a new legal framework founded on the duty of sincere cooperation.