The call for an appellate mechanism in international investment arbitration is located within a broader debate concerning judicial review of international adjudicatory decisions. Existing avenues of setting aside, refusal of enforcement and annulment offer little solace because grounds for intervention generally do not extend to the merits. This article compares three international review mechanisms: the ad hoc annulment procedure of the International Centre for the Settlement of Investment Disputes, the Appellate Body of the World Trade Organization and the Optional Appellate Rules of the American Arbitration Association. Against this cross-regime background, the potential of a (multilateral) appellate tribunal in investment disputes, as advocated by the European Commission in its proposal for an Investment Court System, is evaluated. Notwithstanding potential disadvantages in terms of additional cost and length of proceedings, an appellate mechanism could nonetheless enhance the coherence, consistency, and, ultimately, (the public perception of) the legitimacy of international awards.