The involvement of amici curiae in investment arbitration is becoming increasingly common in disputes that raise public policy considerations. Amicus curiae is a useful tool to promote the transparency, accountability, and openness of the arbitration system, but also to help tribunals in rendering better awards. The 2006 amendments to the ICSID rules facilitated the participation of civil society in the proceedings but did not go far enough in ensuring a full application of the public participation principle. A review of the ICSID jurisprudence shows that the extension of participatory rights of amici curiae beyond the presentation of written submissions has been timid. The efficiency of amicus participation without access to arbitral documents and hearings is doubtful. The ICSID Arbitration Rules should be reformed in line with recent developments so as to endow non-disputing parties with the necessary tools to perform their function properly.