Customary principles regarding contracts concluded with foreigners are deeply rooted in ancient rules of customary international law governing the treatment of aliens. The recent intense development of investment arbitration has been a golden opportunity to further clarify how these principles have developed and are applied to current contractual practices. Admittedly, there is now in general international law a set of principles recognizing and taking into consideration the distinct features of State contracts, beyond the specific rules regarding the protection of foreign investment. Moreover, this process of internationalization has progressively been enriched by principles that are more familiar to international commercial arbitration, and in particular those related to the "order public transnational". However, the scope of these principles is limited and it would be exaggerated to suggest the existence of a consistent and complete international law regime of State contract. Some differences remain as regards issues for which the law applicable to the contract or the doctrinal conception of State contracts have an impact on the principles and rules applicable by the tribunal.