The purpose of this thesis is to analyse the concept of nationality for corporate investors under the Convention on the Settlement of Investment Disputes between States and Nationals of Other States (,ICSID Convention'). Pursuant to Article 25 of the ICSID Convention, ICSID arbitration tribunals have jurisdiction to resolve legal disputes arising out of an investment between an ICSID Contracting State and a national of another ICSID Contracting State, provided the disputing parties have consented, in writing, to submit their disputes to the ICSID jurisdiction. Article 25 requires the corporate investor to hold a certain nationality, but it does not lay down a method for the determination of that nationality. It is argued in this thesis that interpretation of the concept of 'nationality' for corporate investors by most ICSID tribunals risk an unwarranted extension of host state consent to arbitrate. This unwarranted extension of consent exposes the respondent host states to claims by an unlimited number of investors, including those that have not obtained the host state's consent to arbitrate required under the ICSID Convention. In the absence of a sound assessment of the jurisdictional requirements, tribunals risk resolving disputes over which they do not have jurisdiction. This raises serious questions on the legitimacy, and validity of the decisions rendered by ICSID tribunals which are not subject to any meaningful review and which often have significant impact on the general public. Against this background, a re-evaluation of the concept of corporate nationality is offered, which aims at mitigating the concerns on legitimacy and certainty that arise from jurisdictional defects. This attempt is guided by a balanced approach to the disputing parties' rights and interests and with a greater emphasis on economic realities rather than formal appearances. In this connection, the purpose and special characteristics of the corporate entity and the exceptional nature of the ICSID dispute settlement mechanism are placed at the centre of the analysis. Finally, it is argued that the 'real seat' theory, found in various civil law jurisdictions, should be central to the assessment off corporate investors' nationality under the ICSID Convention.