The international arbitration of investment treaty disputes between foreign nationals and their host States is commonly supposed to restrict the exercise of sovereignty. Yet several expressions of territorial jurisdiction have been identified by investment tribunals as presumptive rights of States, which formally derive from custom and are accordingly considered in their interpretation and application of treaty standards. Arbitral reasoning has thereby modernised the general international law on what a State can do in the economy. And what it cannot. This thesis highlights a positive contribution of investment treaty arbitration by showing how key cases and lines of authority have reformed the limits and content of three rights of States in the economy: expropriation, taxation, and regulation. It is organised in two parts and nine chapters. Part I lays a methodological foundation for approaching the notion of economic sovereignty as a bundle of rights under customary international law, which are routinely applied and thereby developed in investment treaty arbitration. Part II examines how the limits and content of three rights have been reformed through the arbitral application of old legal forms to modern economic measures, underpinned by several past projects in the international legal field and a general distinction between governmental authority and commercial activities. These core rights may be consensually limited by any contractual promise but must be integrated in investment treaty interpretation. For any public purpose, each State enjoys the presumptive rights: to take the property of foreign nationals with an offer of compensation at fair market value (expropriation); to make a compulsory exaction of money by law, whether or not for the predominant purpose of raising revenue but without any reciprocal benefit to the payer (taxation); and to adopt, enforce, or amend a measure creating domestic rights and obligations, subject to a circumstantial standard of reasonableness (regulation).