The establishment of the new dispute settlement system in 1995 is undoubtedly one of the major achievements of the WTO compared to the GATT. However, there are still areas that need improvement. This study focuses on one of the controversial reform issues: the participation of non-state actors in WTO dispute settlement, in particular through submission of amicus curiae briefs. This book outlines the legal bases for such forms of participation, the pertinent practice of WTO panels and its Appellate Body and gives an assessment of the positions that WTO Member States hold on this issue. Important policy considerations surrounding this topic are also analyzed. Further the study provides a comparative analysis of non-state actor participation in international investment arbitration.