The primary objectives of this thesis are to clarify and distinguish the existence of regulatory expropriation from other types of indirect expropriation and to propose alternative ways to rescrutinise regulatory expropriation in light of its establishment and justification in international law. In pursuing the first objective, the research delves into certain national, regional, and international contexts, identifying the key features of regulatory expropriation – for instance, regulatory autonomy and regulatory interference – and conducts an extensive exploration of the relevant principles of international law. This exploration concentrates especially on the doctrine of police power and on principles that can contribute to an elaborated application of the doctrine for the identification of regulatory expropriation. With regard to the second objective, the research examines the principle of necessity in international law that functions to preclude the wrongfulness of a state’s act and also takes into account the interacting relationship between customary international law and bilateral investment agreements. In addition, the research goes a step further by means of its analysis of the necessity exception in WTO law. This thesis puts forward the conclusion that an arbitral approach to bona fide regulatory expropriation that can be justified in international law, if it is based on the elaborated application of the doctrine of police power and on the application of the principle of proportionality within the framework of the ends-means and the cause-effect, will be more desirable for investment treaty arbitration given that arbitration is a type of public law adjudication.