<p>This thesis addresses the problem of treaty shopping in investment treaty law. It seeks to illustrate how the problem stems from, and can in part be resolved by, the concept and definition of corporate nationality. It explores whether, and if so how and what, limits ought to be placed on the manipulation of nationality for the purpose of gaining investment treaty protection, to enable a principled basis to utilise nationality to prescribe the extent of rights and obligations in investment treaties. The importance of nationality requirements in investment treaties cannot be overstated—the definition of “investor” in any treaty defines which entities are entitled to substantive protections contained in the treaty for the benefit of states and investors alike. Entities making an investment need to know whether, and if so how, they can structure their investment to achieve protection of applicable investment treaties. Investors who have suffered damage need to know whether they are entitled to make a claim. States need to appreciate the extent of their potential obligations. Many investment treaties define qualifying investors in a broad way that includes any entity incorporated in a contracting state. Putative investors, including those from third states, or nationals of the host state of the investment, seek to come within the relevant definition, often by insertion of an intermediary company incorporated in the desired home state into the ownership chain of the investment. This thesis challenges the view that fulfilment of formalities set out in an investment treaty is sufficient to qualify as an investor where there is no substance behind the corporate form. To some degree, states and investment treaty tribunals have tried to abrogate treaty shopping by manipulation of corporate nationality by reference to the international law concept of genuine connection with the claimant’s state of incorporation, or by way of imposition of criteria for nationality based on the nationality of the corporate entity’s controller or proof of substantial business activity in its state of incorporation. The majority of investment treaty tribunals, however, have eschewed efforts to imply a substantive test or check on the attribution of nationality beyond literal fulfillment of nationality criteria. This thesis promotes a purposive approach that requires fulfillment of express treaty criteria for nationality, but also subjects the claimant to a substantive economic reality check in which the inquiry is to determine the reason for existence of the corporate claimant in relation to the relevant investment. Such an approach is required by an interpretative methodology that gives equal weight to the four tenets of art 31(1) of the Vienna Convention: ordinary meaning, good faith, context and object and purpose. If a corporate entity exists primarily to procure treaty rights, then it is not a bona fide investor consistent with the object and purpose of investment treaty jurisdictional provisions, even if it complies with the ordinary meaning of the express formal nationality criteria. If, however, it meets any express criteria and has a genuine ulterior commercial reason to exist in the ownership structure of the investment, then it qualifies as an investor entitled to the protection of an investment treaty. The approach promoted by this thesis is derived from the treaty shopping antidote crafted by municipal courts assessing the bona fides of corporate applicants for tax relief under double tax treaties. In addition, the thesis analyses municipal law regarding piercing the corporate veil, the law of diplomatic protection, and analogous jurisdictional concepts in investment treaty law including the application of the principle of abuse of right, and identifies that underlying all these areas of inquiry is the central question of the purpose, or commercial reason to exist, of the relevant corporate entity. Finally, this thesis demonstrates how a substantive approach can be applied in a principled and reasonably certain way. The use of corporate structures by foreign investors to procure rights under favourable investment treaties (treaty shopping) threatens to undermine the legitimacy of international investment treaty arbitration. Simon Foote QC's research illustrates how the problem stems from the concept and interpretation of corporate nationality criteria at international law. It promotes a new way to distinguish bona fide foreign investors by looking to the commercial purpose of corporate entities in relation to the relevant investment. It illustrates how that approach derives from analogous concepts in international and municipal law and how it can be implemented by states and investment treaty tribunals.</p>