Comparison shows that membership in the European Union (EU) – but already preparation for accession – raises numerous and specific constitutional challenges due to the specific features of the EU legal system. Only in few cases such adaptation can be achieved by a wide interpretation of existing constitutional provisions; most often it will require amendments to legislation and even to the Constitution itself in order to clarify and regulate fundamental questions thus creating legal certainty. Thus, almost all EU Member States have amended their Constitutions, either on occasion of Treaty revisions or in view of their accession to the EU, in order to meet the requirements resulting from their integration into the EU legal system.