The thesis’ aim is to scrutinize the tug of war or rather the tension between national social security systems and the EU legislation as well as the European Court of Justice’s case law in this field. There are, as will be shown, conflicting interests between the member States’ internal systems which are based on solidarity and the EU which is based on the free movement of workers. These aims are not necessarily possible to compromise upon. Furthermore the European Court of Justice has taken a more active approach in the social security field in the late 1990’s and onwards. The Court has, in some aspects, gone further than just interpreting the various EU instruments when introducing the Primary Law when delivering its judgments. There is thus an insecure feeling amongst Member States that the European Court of Justice might have a hidden agenda; to harmonize the social security laws. Is there some truth behind these fears?