According to the European Union's (EU's) principle of subsidiarity, responsibilities of any kind should always be allocated to the lowest level possible. When applying this to the EU's economic union, one might assume that the decentralization of trading structures as well as the ensuring of national economic freedoms are basic necessities for a reliable implementation of the concept of subsidiarity. Nevertheless, with view to the sustained enlargement of the European Single Market, it seems that the centralization of trading structures has not lost its attractiveness yet. The permanent progress in European trade integration raises the important question whether there may be some optimal degree of economic integration in the sense of subsidiarity and, if so, how this optimal level of integration might be determined. Answers to this burning issue will be of fundamental importance for future European policy making. As long as it is not clear-cut whether the principle of subsidiarity is of an integrationist or a more anti-integrationist nature regarding deeper trade integration, an adequate implementation of the subsidiarity principle appears impossible for policymakers at both the national and the European level.