The present chapter examines whether, and if so to what extent, customary international law applies within the internal legal sphere of the EU, that is, between EU member states within the EU law’s scope of application as well as in their legal relations to the EU. In essence, the relevance of customary international law within the EU’s internal legal sphere is about the EU’s assertion of autonomy and self-containment that has been unfurled by the CJEU. The analyses of key areas of customary law (e.g. diplomatic relations, sovereign immunity and equality, rules of responsibility) reveal a complex picture of the its rules’ relevance within the EU. They play a more tangible role in the relationship between EU member states, first and foremost in sovereignty-related areas, than in member states’ legal relationships to the EU. Nevertheless, the present chapter shows that despite being a ‘new legal order’, the EU treaties still constitute a subsystem of public international law, albeit one which manifests typical characteristics of self-containment.