This chapter assesses the relationship between the European Court of Human Rights (ECtHR) and domestic and international legal systems. With the ratification of the European Convention on Human Rights (ECHR), the Member States accept to be bound by final judgments of the Court and to implement them in their domestic legal systems. The Convention system does not make any difference as to the set-up of the national legal system or to the hierarchical position accorded to the Convention in national law. This is in line with a purist international law perspective, summarized in Article 27 of the Vienna Convention of the Law on Treaties: ‘A party may not invoke the provisions of its internal law as justification for its failure to perform a treaty.’ However, from the constitutional law perspective of the Member States, the situation is much more multi-faceted and complex. While it is generally accepted that the Court's judgments are binding and have to be implemented, the relationship between the Convention and the national constitutions as well as between their respective guardians, the Court on the one hand and national constitutional or supreme courts on the other hand, is not seen as one-way and hierarchical, but nuanced and differentiated. Implementation of judgments is accepted to be a duty, but not necessarily without exceptions. The chapter then considers the relationship between the ECtHR and the European Court of Justice (ECJ).