One of the sources of international law listed in Article 38(1) of the Statute of the International Court of Justice is international custom, as evidence of a general practice accepted as law. This paper examines the contemporary role of custom in the present context of the proliferation of bilateral treaties on investment protection. It looks at three traditional reasons invoked by scholars to explain the continuing relevance of custom in contemporary international law. There exists also another cause for the importance of custom in investor-State arbitration, because international law is the applicable law in the majority of arbitration disputes. Therefore, the paper examines the question of the application of international law (including customag rules) by tribunals through the different ways arbitration claims can be introduced by foreign investors.