This chapter discusses how the CJEU invokes and applies substantive customary international law rules within the EU legal system; to which extent they are relevant to this system: whether the interpretation of these rules by the CJEU conforms to the traditional rules of interpretation in international law; and whether the Court pursues a specific strategy in doing so (and whether there is coherence and/or certain patterns to its jurisprudence in this respect). The substantive customary international rules used by the CJEU and thus further assessed here are those governing ‘territory’, ‘territoriality’ and ‘extra-territoriality’; substantive customary rules of the VCLT (other than interpretation rules); and lastly, some of the most fundamental rules of international law (such as the prohibition of the use of force and the right to self-determination).