In the last several years, the idea that international investment arbitration should become more transparent has gained wide acceptance. Until quite recently, however, disputes between states and foreign investors were considered private as a matter of law. The main implication of this approach was the confidentiality of the proceedings. While this paradigm is still dominant in the field of international commercial arbitration, it has been substantially weakened in the area of investor-state arbitration.1 It is now commonplace to make arbitral awards and/or parts of the proceedings public. This is perfectly consistent with the subject matter of investor-state disputes, which in many cases raise issues of public concern. Thus, in a way, one can say that real life has entered into the legal realm. The issue of amicus curiae intervention in international investment disputes offers a good illustration of how this change came about.

In a number of investor-state disputes, several non-governmental organizations (NGOs) have successfully drawn upon the public character of trade and foreign investment disputes to gain access to the proceedings as amici curiae (meaning “friends of the court”). They have raised a variety of arguments, ranging from jurisdictional issues to environmental and human rights considerations. This chapter analyzes how the practice of amicus intervention spread from interstate adjudicatory bodies to investor-state arbitrations, focusing on certain developments under the International Centre for the Settlement of Investment Disputes (ICSID).