This article employs a mixed methodological approach to evaluate non-disputing party (NDP) participation by means of amici curiae submissions in investment disputes settled by the International Centre for the Settlement of Investment Disputes (ICSID). The work presents and analyzes statistics on NDP participation in ICSID cases in order to investigate whether amicus submissions are a suitable vehicle for the incorporation of the views of NDPs, and whether they contribute to the development of international investment law and policy. The article argues that amicus briefs are a welcome addition to the ICSID arbitral process in theory, having contributed to the enhancement of the rule of law in investment arbitration through increased transparency. However, the level of NDP participation that amicus briefs provide within ICSID is minimal, and the extent to which they may affect the final outcome of the case is very much at the discretion of the individual tribunal. In light of this, and recognizing previous research that has noted the potential benefits of amici curiae submissions in dispute settlement processes (investment and otherwise), this article will go on to suggest reforms to enhance the participation of NDPs in ICSID arbitration. This will, in turn: contribute to the development of international investment law and policy; increase its integration with other branches of public international law (namely human rights); and enhance transparency and respect for the rule of law.