More than fifty investor-state arbitration claims have been filed by foreign investors against the Republic of Argentina due to the country's adoption of measures to mitigate the consequences of a severe financial crisis that struck the country in the early 2000s. Argentina invoked the Non-Precluded Measures (NPM) clause in the U.S.-Argentina Bilateral Investment Treaty (BIT) and the necessity defence in customary international law as its defense in these arbitrations. As a result of taking divergent approaches to interpreting the NPM clause in the U.S.-Argentina BIT, the tribunals reached inconsistent decisions on Argentina’s liability for damages incurred by foreign investors, which intensified the legitimacy crisis in the investment arbitration regime. Consequently, the tribunals’ approaches to interpreting the nexus requirement of the treaty NPM clause (the "necessary for" term) caused a fierce academic debate among scholars. This thesis studies the issues related to the inconsistent interpretation of treaty NPM clauses and the customary necessity defense in the investment arbitration regime. It presents a detailed examination of the necessity defense in customary international law and treaty NPM clauses through the lens of regime theory. By applying relevant concepts of regime theory, such as regime formation, regime attributes, regime consequences and regime dynamics, this work explores the origins and evolution of the necessity doctrine, and provides a comparative analysis of the attributes, structural elements and the consequences of invoking the customary necessity defense and treaty NPM clauses. This thesis analyses the interpretative issues in the Argentine cases, and based on the dynamics of developments in the practice of states, it arrives at concrete proposals that will contribute to the coherent practice of investment arbitration tribunals in interpreting treaty NPM clauses. By applying the concept of interaction of regimes, this thesis provides a comparative analysis of tests suggested by scholars for interpreting Article XI of the U.S.-Argentina BIT. It examines whether the interpretative testsmargin of appreciation, proportionality and less restrictive meansused by dispute settlement bodies in other specialized treaty regimes have the potential to serve as an optimal standard for interpreting Article XI. This work explains the contents of these tests and inquires as to the advantages and criticisms related to their application in the investment arbitration regime. This thesis further advances the argument that the interpretation of treaty NPM clauses (Article XI of the U.S.-Argentina BIT) should be performed with strict adherence to the general rules of interpretation as established under Article 31 of the Vienna Convention on the Law of Treaties (VCLT). Specifically, it argues that in cases when tribunals fail to define the meaning of a treaty provision under Article 31 (1) and (2) of VCLT, they should not look for guidance from other specialized treaty regimes, but rather, must have recourse to general international law, specifically, customary rules of international law. As a methodology for performing this interpretation, this thesis proposes to apply a systemic integration approach through operationalizing Article 31(3)(c) of VCLT. Furthermore, this thesis advances the argument that the interpretation of the only means requirement of the customary necessity defense (Article 25 of Articles on the Responsibility of States) does not accurately reflect the contemporary customary rules on necessity. Thus, by applying the concept of regime dynamics, it proposes to reconceptualise the interpretation of the only means requirement through incorporating the elements of a more progressive version, which is found in the international trade regime. Unlike the scholars who rejected the application of the customary necessity elements, and proposed the direct importation of the LRM test from the international trade regime to interpret Article XI, this thesis proposes a different approach to taking advantage of the WTO jurisprudence. Specifically, it argues that WTO jurisprudence can be incorporated into the investment regime indirectly by serving as a source from which we can identify the development of state practice in examining the "only means" nature of state measures adopted in emergency (necessity) circumstances. It is contended that such state practice represents a more progressive and practical approach to interpreting the only means requirement of customary necessity defense, and thus, should be incorporated into the interpretation practice of investment arbitral tribunals.