in this volume by Jarrod Hepburn is of growing importance in both theoretical and practical terms, as reflected in both investment treaty awards and, increasingly, international investment agreements (‘IIAs’). The 2015 decision of the majority in Clayton/ Bilcon v Canada, according to the dissenting arbitrator Professor Donald McRae, controversially equated a breach of Canadian law with a breach of the fair and equitable treatment standard under chapter 11 of the North American Free Trade Agreement. In late 2016, the tribunal in the long- running dispute of Pac Rim Cayman v El Salvador, constituted under the ICSID Convention on the Settlement of Investment Disputes, confirmed its jurisdiction to rule on a matter arising under Salvadoran investment law. After hearing considerable expert evidence on the central issue of the correct interpretation of Salvadoran law, the tribunal found no breach of either Salvadoran or international law. Perhaps partly in response to these kinds of disputes, countries such as Colombia, Canada and the European Union have attempted in some of their IIAs to limit the jurisdiction of investment tribunals in determining the legality of a measure under the domestic law of the host State, including in the now signed Comprehensive Economic and Trade Agreement between Canada and the European Union. These kinds of attempts by States to curtail the jurisdiction and power of investment tribunals with respect to domestic law appear consistent with the traditional view of domestic law as a question of mere ‘fact’ rather than ‘law’ for the purposes of public international law. However, as Hepburn explains, that traditional ‘rule’ is subject to uncertainty, exceptions and blurred edges, both in international law in general and in international investment law in particular. Some IIAs specifically identify domestic law as part of the applicable law for an investment treaty tribunal, while in other circumstances tribunals must in any event turn to domestic law to delineate contractual or property rights, in the absence of relevant international law rules. In the World Trade Organization (WTO), a parallel realm of international economic law, the Appellate Body has also refused to treat domestic law purely as fact, by insisting on its capacity to review WTO panels’ examination of such law.