Is privity of contract the reason why investor-state dispute settlement (ISDS) is open to critics, or could it contribute to solving the system’s legitimacy crisis? Privity of contract essentially means that a subject must be a party to a contract, in order to acquire rights and assume obligations, to sue and be sued under that contract. Privity of contract came to land on the shores of ISDS and this has at least on one occasion been described as an ‘original sin’. Arbitral tribunals often need to decide whether they have jurisdiction in cases where a party to the investment contract is not the claimant but a related entity, or not the central government, but a state agency or state-owned enterprise. In light of the deep interconnection between, on the one hand, the criticism today surrounding investment treaty arbitration – be it called judicial activism and regulatory chill, or be it called abuse of law and indirect claims – and, on the other hand, the domains where privity of contract applies, this book’s original and far-reaching analysis clearly lays out, via an in-depth examination of relevant case law, a possible use of the doctrine that can contribute to leading ISDS out of the crisis. The study’s conclusions respond with thoroughly researched authority to such key questions as the following: In which domains of international investment arbitration does the notion of privity of contract operate, and with what effects? How are states and arbitral panels reacting to the persisting unresolved issues raised by the increasing pertinence of this legal doctrine? What solutions are advisable in the midst of the current criticisms surrounding ISDS? The author finds that the doctrine of privity of contract finds application in heterogeneous scenarios, from decisions on jurisdiction where there are forum selection clauses in investment contracts or fork-in-the-road provisions in investment treaties, to consolidation, counterclaims and umbrella clause claims. She proposes a flexible interpretation of the doctrine of privity of contract as a guiding principle arbitral tribunals should consider along with other factors (inter alia the tightness of the relation between the investor and its subsidiary and the host state’s involvement in the organization and function of agencies or state-owned enterprises). The book’s thorough and extensive examination of investment arbitration case law draws comparisons with other international adjudicatory bodies and identifies the most actual and compelling unresolved legal issues. Appendices include lists of many of the arbitration cases, international judgments and national judgments discussed. As a constructive contribution to the current debate, this enquiry is an extraordinary achievement. No other study has conducted such thorough research on the application of privity of contract in investment treaty arbitration. It will be of great interest to arbitration lawyers, arbitrators, foreign investors, host states and scholars in all areas of international arbitration and dispute settlement.