The impact of the European Union on Social Security is quite complex and enigmatic. At the starting point, there is a genuine paradox: whereas the construction of a large and unique market supposes, among many others prerequisites, the harmonization of social security systems, this harmonization is left to the good will of the member States since Social security is not truly within the competence of the Union. In these conditions, it is quite obvious that a thought and organized harmonization is absolutely unreachable. Beyond the threat of systems of social protection in the European Union contracting, the issue arises of their restructuring and alignment. Given the common challenges they all face (ageing populations, family instability, un...