In 2015, the International Tribunal of the Sea (ITLOS) issued its first Advisory Opinion as a full court on the request of the Sub-Regional Fisheries Commission (SRFC). In its reply to the first of the questions put to it, the ITLOS held that flag States have the ‘responsibility to ensure’ that their vessels are not engaged in Illegal, Unreported and Unregulated (IUU) fishing in the Exclusive Economic Zones (EEZ) of SRFC Member States. As this obligation derives directly from the United Nations Convention on the Law of the Sea (UNCLOS), several commentators referred to an obligation for UNCLOS States Parties to ensure that their ships do not conduct IUU fishing in foreign EEZ in general, and not only in the EEZ of SRFC Member States. While such a generalization is desirable for the achievement of sustainable fisheries internationally, the use of term of ‘IUU fishing’ should nevertheless be avoided in this context. Indeed, this concept only has real functional applicability for High Seas fisheries, whereas its applicability is limited in the context of EEZ fisheries. Furthermore, the UNCLOS does not contain a definition of IUU fishing, which means that its definition must be found elsewhere in order to be applicable to UNCLOS Member States. As there are only a limited number of situations where it is the case, it is preferable to speak of a flag State responsibility for illegal fishing and illegal fishing-related activities – rather than for ‘IUU fishing’– in order to achieve better generalization.