This Article identifies the problems of an Appellate Mechanism for ISDS Tribunals in relation with its possible benefits. We propose the inclusion of certain design features to improve the working of an eventual Appellate Mechanism and help mitigate problems related to procedural, conflict resolution, and substantive concerns. We finish by identifying the most central problems with a possible Appellate Mechanism, which helps to narrow down options within the ongoing reform process at UNCITRAL. Overall, we illustrate how institutional choice is always contextual and that all institutional options are imperfect and subject to important trade-offs.