AbstractRegional courts have synthesized, articulated, and elucidated certain principles of law that influence the development of international investment law. The contributions of NAFTA Chapter 11 dispute settlement framework and European Court of Human Rights (ECtHR), in particular, have been outstanding. For instance, NAFTA jurisprudence has guided investor-state dispute settlement (ISDS) tribunals through influential precedents. Similarly, the doctrine of proportionality and the margin of appreciation doctrine which emerged from the ECtHR jurisprudence have become embedded in international investment law. Indeed, given the unique contributions of regional courts and their rapid proliferation, it can be predicted that they will play even more significant roles in the future development of principles of international investment law. Arguably, such emergent principles should be subjected to a prior scrutiny and filtering by ISDS institutions as a precondition to full incorporation into international investment law to foster their legitimacy and credibility.