The aim of this book is to examine how EU law relates to and impacts on the national social security systems of the Member States. It asks three key questions. Firstly, it looks at how the internal market and its developments have eroded Member States” sovereignty over their social security systems, despite the fact that the EU has limited competence in the field. It then explores, secondly, how the Union Citizenship and, thirdly, the Charter of Fundamental Rights has affected the coordination of these regimes.