A reform proposal for investor-state arbitration has been debated by UNCITRAL Working Group III since 2017 and many options have been considered including state-to-state arbitration. In this background, this book which was originally submitted as a Dissertation for a Doctoral Degree at Saarland University is presented to you. The topic for the work was inspired by the situation that while we had state-tostate arbitration as an option since the first bilateral investment treaty, just a handful of cases had emerged, a curious situation since the world has seen more than 900 investor-state arbitration disputes based on the same treaties.