The concept of a European Constitutional Space has been used in legal scholarship to describe a common space of constitutionalism where national and international constitutional guarantees interact to maintain the common constitutional values of Europe. This concept has not yet been tested in a case where the constitutional order of a Member State of the European Union seems to develop systemic deficiencies. The present volume aims to assess recent constitutional developments in Hungary and Romania, as well as the interplay of national, international and European constitutionalism which react to the loopholes in national constitutions. Accordingly, a core part of the volume is an in-depth analysis of the situation in Hungary and Romania. Based on that, the volume offers an account of the different reaction mechanisms of the European Union and of the Council of Europe. Beyond a detailed stock-taking of these mechanisms, their legal and political frameworks are explored, as well as different ways to extend their reach. In this way, the volume contributes to a little studied aspect of European constitutionalism.