The principle of proportionality in international investment arbitration

International investment law is a dynamic and multidisciplinary branch of law that has gained increasing importance due to globalization and regulates the legal relations between states and foreign investors. One of the fundamental objectives of this field is to establish a fair balance between the rights of foreign investors and the sovereign powers of host states, thereby contributing to the sustainability of the investment environment within a legal framework of security. In this context, international investment arbitration stands out as one of the most important dispute resolution mechanisms that protects the rights of investors and subjects the investment regulations of states to review in accordance with certain legal principles. This study examines in detail the role of the principle of proportionality in investment arbitration within the framework of the fundamental standards of protection in international investment law. In this sense, it is possible to say that the principle of proportionality plays a role in many areas, such as indirect expropriation, the interpretation of the standard of fair and equal treatment, the determination of compensation for which states are responsible, the definition of investment, and the interpretation of exception clauses. The principle of proportionality is recognized as a critical criterion in the legal review of states' regulatory actions and interventions targeting foreign investments. While states have the authority to adopt various regulatory measures for the public interest, the arbitrary, excessive, or disproportionate exercise of this authority is incompatible with investors' expectations of legal certainty and may give rise to disputes before international investment arbitration tribunals. Keywords: International Investment Arbitration, Proportionality, International Arbitration, Protection of International Investments, Foreign Investments