The article analyzes the legal standing of amici curiae in international fora, focusing on the recent decisions rendered in the context of international trade and investment disputes. After a short review of the idea of amicus curiae as it developed in the common law tradition, the author discusses the legal treatment given to amici curiae by a number of international courts and tribunals, including the International Court of Justice, the European Court of Justice, the European Court of Human Rights, the Inter-American Court of Human Rights, the WTO Appellate Body, NAFTA (UNCITRAL) panels and ICSID arbitral tribunals. Drawing upon orders issued in two recent ICSID cases, in May 19 2005 and March 17 2006, respectively, the discussion emphasizes the fundamental tension underlying the intervention of amici curiae in investor-state arbitral proceedings, namely the conflict between the traditional consent-based legitimacy and the increasing need for public involvement.