The topic of transparency in international investment arbitration is gaining increasing attention. This in-depth commentary analyses the UNCITRAL Rules on Transparency in Treaty-Based Investor-State Arbitration, one of the most recent and innovative developments in international law. Focusing on the application of these rules, contributors analyse the issue of transparency in investment law more broadly and provide in-depth guidance on how to apply the UNCITRAL transparency rules. Chapters encompass all treaty-based disputes between investors and state, examining the perspectives of disputing parties, third parties, non-disputing state parties and arbitral tribunals. The contributors each have a strong background in investment arbitration, in both professional practice and academia. This commentary will be of interest to all actors involved in investment arbitrations, especially practitioners, counsels, NGOs and scholars in the fields of international law, commercial arbitration and investor-state arbitration.