Illegal fishing in the Exclusive Economic Zones [EEZs] of developing coastal States is an urgent problem for the marine environment, global food security, and local economies. While past academic debate has predominantly focused on obligations of flag States to tackle so called IUU-fishing in the High Seas, the recent request for an advisory opinion submitted by the Sub-Regional Fisheries Commission to the International Tribunal for the Law of the Sea (ITLOS, Case No. 21) has drawn attention to the fisheries regime of the EEZ. This article argues that the primary responsibility for fisheries management in the EEZ rests on the coastal State and that, so far, flag States have no obligation under customary international law to exercise their jurisdiction and control over vessels flying their flag which fish in the EEZ of other States. The article first gives an account of coastal State regulatory and enforcement jurisdiction. It outlines recent developments of the law by drawing on the jurisprudence of the ITLOS, particularly the recent M/V “Virginia G” Case. Further, the article undertakes to identify potential flag State obligations to combat illegal fishing in the EEZ. To that end, it provides an in-depth analysis of relevant binding and non-binding legal instruments such as the 1982 UN Convention on the Law of the Sea, other multilateral treaties, bilateral fisheries treaties, and relevant soft-law instruments of the Food and Agriculture Organization of the United Nations. The article also discusses the relevance of principles of international environmental law. Next, the article analyzes the nature and scope of potential flag State obligations, qualifying them as obligations of due diligence. Finally, the article concludes that, de lege lata, no persuasive evidence of established flag State obligations exists. The author suggests that the situation should be remedied by a new, fully binding legal instrument.