Through a comprehensive interview regime and archival research spanning three organizations, this study of UNCITRAL and international investment law examines: (i) how UNCITRAL transitioned from its private law origin to become the global centre of ISDS reform; (ii) what impact UNCITRAL and its Arbitration Rules has had on the development of the ISDS system; and (iii) what the UNCITRAL example reveals about contemporary public international lawmaking, institutional choice, and venue competition.