This contribution seeks to analyse the contribution of the European Court of Justice (ECJ) to the European Union’s duty to contribute to the strict observation and development of international law (Article 3.5 TEU). The introduction shows that the ECJ’s hands were largely tied by some basic provisions of the Treaties on the question of the hierarchy between international law and Union and national law, as well as on the relation between Member State agreements pre-dating Union law and Union law itself. Hence the contribution concentrates on the ECJ’s view of two issues: (1) the reception of different forms of international law (treaty law and customary law) in Union law and (2) the position of Union primary and secondary law in relation to these different forms of international law. Ways of dealing with these issues, such as harmonious interpretation and direct effect, are briefly touched upon. Finally, brief sections are devoted a few special issues, such as the place of agreements concluded by the Member States in areas of Union competence, without the Union being capable of also adhering to them.