Would an observer from an outside world look on the European Union the picture would be one of a obscure rag rug, an organisation monstrum simile, as Samuel Pufendorf put it with regard to the 17th-century Holy Roman Empire. Nevertheless to the great surprise of this obeserver (and not only to his suprise) the system works, more or less. This surprise is justified by the fact that in this community several states joined under the name Union, though these states could not be more different to each other with regard to their polity: federal, semi-federal, regionalized, unitary; parliamentary or presidential; monarchy or republic; constitutional or, in a formal sense, non-constitutional; monistic or dualistic; with and without plebicitarian elements; with and without a constitutional court. Additionally quite different legal and constitutional traditions represent a considerable burden to any futher integration.However, the moment those states began to build a Union, starting from such different points, things developed complicated. The reason is that the Union accumulated power and competences to an unknown scale, interferring with domestic politics ever more. An integration process accelerated by this leads us to the question: Is a heteregenous Union like this workable at all, if its members lack a minimum standard of substantial similiarity? Obviously it is not. As the paper intends to show, using the United Kingdom as an example, the integration process would have come to an early end if the power of the community law had not have led to a subliminal Europeanization of the legal and constitutional system of Great Britain though the political leadership of the country was adamant to preserve the country's old and honourable constitution.The necessity of a legal-constitutional concordance, that is a prerequisite to a workable Union, however, is in contrast to demands from the political arena that Europe must preserve its diversity, must remain a Europe of nation states. Consequently the opposition against a further integration grows in proportion to its progress, because this process does not longer affect only some marginal provisions on economic policies, but all legal spheres that regulate the normal life of every subject of the union without being legitimated appropropriately by a democratic decision-making process. This leads to tensions between the political demand for diversity and the necessity of harmonization that could represent a considerable threat to the Union