In focus in this chapter is the 2015 Advisory Opinion on Illegal, Unreported and Unregulated (IUU) Fishing rendered by the International Tribunal of the Law of the Sea (ITLOS) concerning obligations of flag states in the exclusive economic zone. Professor Chircop explains the significance of the issues considered by ITLOS in the context of the United Nations Convention on the Law of the Sea (UNCLOS), as well as with reference to international maritime, environmental and fisheries law. He discusses the ITLOS understanding of the responsibilities and liability of flag states in response to the questions posed to it, and reflects on the significance of the Advisory Opinion and the extent to which it has changed the understanding of the due diligence duties of flag states today. He concludes with observations on the potential effects of the Advisory Opinion on maintaining UNCLOS as a living constitution.