Amici curiae are individuals or organisations who do not have the right to participate in the dispute as parties but want to intervene because the outcome of the proceedings may affect their interests. The participation of amici in investor-state arbitration has been justified as a useful tool to pursue different interests, inter alia, the promotion of greater transparency, accountability, and openness of this dispute settlement mechanism. However, opening up investment arbitration to the participation of non-disputing parties may raise several concerns, namely as regards the identity and interests pursued by the so-called ‘friends of the tribunal’. This chapter analyses the provisions of TPP’s Chapter 9 on amicus curiae intervention and discusses to what extent they balance the perceived benefits and potential drawbacks of this mechanism of public participation in investor-state arbitral proceedings. The social acceptance of the TPP will depend, to a large extent, on whether it offers solutions that effectively tackle the criticisms that have been thrown at investor-state arbitration, especially those that relate to a perceived lack of transparency and public participation. However, this goal can only be truly achieved if amicus curiae participation creates added value and does not undermine the purpose of peaceful and orderly settlement of investment disputes.