The International Bar Association (IBA) Investment Arbitration Subcommittee published a report on third-party participation (TPP) in investor-state dispute settlement (ISDS), examining the increasing role of amici curiae and non-disputing treaty parties (NDTPs) in investment arbitration. The report finds that TPP has become a common feature of ISDS, with applications rising significantly over the past two decades, particularly in ICSID proceedings, and a majority of applications being granted. It reviews the legal frameworks governing TPP, including treaty provisions and arbitration rules, and identifies inconsistencies in how tribunals assess applications, disclosure requirements, party involvement and procedural safeguards.