Investor-State Arbitration describes the increasing importance of international investment and the necessary development of a new field of international law that defines the obligations of host states and creates procedures for resolving disputes. The authors examine the international treaties that allow investors to proceed with the arbitration of their claims, describe the most-commonly employed arbitration rules, and set forth the most important elements of investor-State arbitration procedure - including tribunal composition, jurisdiction, evidence, award, and challenge of annulment. The authors trace the evolution and rapid development of the field of international investment, including the formation of the International Center for the Settlement of Investment Disputes (ICSID), and the more than 2,000 bilateral investment treaties, most of which were entered into in the last twenty years. The authors explain how this development has led to far greater certainty for foreign investors in dealing with their host countries, as well as how it has incentivized growth in international trade and commerce.