This thesis examines how linguistic diversity is legally regulated in the European Union by offering a comprehensive overview and analysis of the different ways in which the European Union, directly or indirectly, regulates the use of languages on its territory and promotes linguistic diversity. Language questions in the European Union are far from being settled, and much unresolved tension remains which necessitates a closer scrutiny of the status quo and the formulation of policy recommendations. The starting point is the examination of the scope of EU competences in this field, and the sort of action that is expected from the European Union. Fundamental issues that seem straightforward yet are not without controversy. As will be shown, the EU lacks a coherent language policy, and its efforts in the field are characterised by isolated actions that are scattered around in the Union’s sphere of influence. Importantly, the thesis does not only scrutinise those tools that are applied by the EU with the explicit aim of protecting and promoting multilingualism, but also those legislative instruments that have a different aim yet unintentionally influence the use of languages in the EU and thereby the entitlements of their speakers, not only in the public but increasingly also in the private sphere. In doing so, the thesis will demonstrate that the regulation of linguistic diversity in the EU happens through a hybrid regulatory model comprising three types of tools: individual rights, obligations derived from EU law, and governance instruments. The language planning models of three national or regional regimes will also be examined in detail, that of the United Kingdom of Great Britain and Northern Ireland, the Republic of Ireland, and South Tyrol, in order to draw inspiration from how language questions are managed at the national level. Finally, the thesis will offer some normative considerations and policy recommendations regarding the EU’s action in language matters, proposing a roadmap which could lead to the establishment of a comprehensive, reliable, and satisfactory EU language policy.