To assess the net benefits of the enlargement of the Union to 25 member states the traditional trade integration theory is not adequate. A more satisfactory approach is an analysis based on dynamic scale economies and specialisation opportunities stemming from the integration of the accession countries. In this perspective enlargement could be a great economic opportunity and will have a net positive effect on the European economy. However, the size of these benefits is not given and is related to the adjustment and reform policies that will be implemented by both old and new member countries after May 1st 2004. Completing the Single Market and getting the Lisbon process back on track should be the central future policy goals of the EU 25.