A Few Observations on the Remaining Fundamental Importance of Customary Rules in the Age of Treatification of International Investment Law - ASA Bulletin View A Few Observations on the Remaining Fundamental Importance of Customary Rules in the Age of Treatification of International Investment Law by - ASA Bulletin A Few Observations on the Remaining Fundamental Importance of Customary Rules in the Age of Treatification of International Investment Law 34 1

This article examines the contemporary role of custom in the present context of the proliferation of BITs which have become in the last decades the most important source of international law in the area of foreign investment. The article will provide a survey of the changing importance of the different sources of international investment law overtime. I will argue that customary rules remain of fundamental importance even in this age of ‘treatification’. First, custom is the applicable legal regime of protection in the absence of any BIT. Second, custom is important in the many instances where BITs make explicit reference to the concept. Third, custom plays a gap-filling role whenever a treaty, a contract or domestic legislation is silent on a given issue. Finally, I will explain the reasons why arbitral tribunals should always take into account relevant rules of customary international law.

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