In recent years foreign investors have used a rapidly expanding network of bilateral and multilateral investment treaties to directly sue states before international tribunals for violations of international law. There have now been hundreds of such lawsuits, with tribunals occasionally granting investors massive damage awards. In the process of resolving these disputes, tribunals announce and apply new rules of law. This brave new world of international investment law (“IIL”) has emerged as one of the most dynamic and controversial areas of international law today. In this Article I argue that the IIL system can be usefully analogized to a domestic-law administrative agency, where significant regulatory power is transferred to expert decision-makers acting on behalf of political principals. Viewing IIL as an agency highlights the IIL system’s major weakness: the lack of sufficient mechanisms of political control by states. Drawing on domestic administrative practice, I suggest reforms designed to enhance control by adapting domestic-law systems of notice-and-comment and the legislative veto. Doing so promises to ensure states a more adequate degree of control over IIL outputs.