This study focuses on the different legal tools that can be used by arbitral tribunals in order to distribute between foreign investors and host States the risks arising from environmental regulation. More specifically, the analysis of these tools aims at clarifying, on the one hand, the host States' room for maneuver in the adoption of environmental regulations and, on the other hand, the regulatory risk undergone by foreign investors whose activities are particulary exposed to such regulations. As this study shows, the main tool used to balance these different interests, in the lights of the circumstances of each case, is the police powers doctrine. However, the application of this doctrine raises some difficulties in those cases where the host State has given specific assurances regarding the regulation of the activities of foreign investors.