A primary goal of the International Law Commission’s work on the identification of customary international law is to offer “guidance to those called upon to identify the existence of a rule of customary international law,” particularly national court judges. The ILC report, however, should be no less useful to participants in international adjudication. The report’s impact might be particularly significant in investment arbitration, which, given the field’s growing importance and impact, could greatly facilitate the ILC’s principal mandate of promoting the “progressive development of international law.”Investment arbitration is proving hugely significant for the development of international law. The United Nations Conference on Trade and Development reported that, at the end of last year, there were 568 known cases brought under international investment agreements, including 57 cases brought in 2013 alone. There have been 98 different State respondents, including the United States, which reportedly is the ninth most frequent respondent State.