This book investigates how constitutionality and internationally accepted principles of human rights influence state action. The first part reviews the limitation of sovereignty in the context of the creation, amendment and interpretation of national constitutions. The following questions are addressed: how far has the process of the internationalization of (national) constitutional law progressed; in how far is there any willingness to accept alien, foreign, or international principles and rules. What underlies the decision by the constitutional organs of certain states to accede to such constitutional migration, and the rejection of such migration by their respective counterparts in other countries? The second part covers the constitutionalization of international human rights standards in the case law of national courts. The author shows that the migration of constitutional ideals is not necessarily a one-way road. The phenomenon of 'reverse globalization' presents itself in Europe. The author concludes that the number of countries that are engaged with foreign and international norms is growing. Whether this will also entail the emergence of a transnational constitutionalism remains to be seen. This book is a thorough comparative study of the use of foreign and international law by national courts. It is of interest to academics in the field of constitutional law and human rights.