Has the performance of UNCITRAL’s mandate evolved over time in relation to the governance of international investments? The chapter argues that UNCITRAL’s activities concerning investment law may be divided in two discrete instants. A first period is marked by an attentive performance of UNCITRAL’s mandate to promote “the progressive harmonization and unification of the law of international trade”, focused only on the private dimensions of international investment transactions. A second period begins recently with UNCITRAL’s involvement in the governance of investment arbitration. In particular, the project of ISDS reform has been undertaken at UNCITRAL under a presumably expanded understanding of its mandate and mission. It is, however, unclear whether this reconstruction of UNCITRAL’s mandate merely reflects a stronger engagement with public international law or an attempt to pursue activities of institutional imagination seemingly geared towards the transnational constitutionalization of the international investment regime. In any event, the new set of tasks conducted by UNCITRAL will require changes to the methods and practices historically developed for the transnational harmonization of municipal commercial law. This chapter aims at contributing to a reflection on the obstacles that UNCITRAL may face in the years to come regarding its evolving role as one of the centres for the transnational governance of international investments.