The legitimacy and effectiveness of the International Criminal Court (ICC) have come under severe criticism in recent years, leading—in part—to states recently exiting the institution. Crucial to the ICC’s legitimacy is its processes and procedures, including the appellate process. Appeals allow for errors to be identified, challenged, and remedied, as well as providing an accountability mechanism for judges and litigants. However, that process has been understudied empirically. This paper theorizes that the fairness of the appellate process can be assessed on several dimensions, including the proper application of the law, efficient administration of justice, rate of affirmance and reversal, and the representativeness and experience of the Pre-Trial, Trial, and Appellate Chambers. Each dimension captures common areas of concern of the appellate process at international courts, like the ICC. The paper also presents new data on all appellate judgments from convictions or acquittals and interlocutory appeals at the ICC up until 2021. The data reveal that, on average, appeals are resolved in 201 days and result in reversals 37 percent of the time. The data suggest that the different legal backgrounds of the judges between the Chambers may lead to more reversals on issues pertaining to state sovereignty, which may undermine legal legitimacy but promotes broader political goals of the Court. Based on the finding, the appeals process at the ICC reflects active judicial oversight with fluctuating reversal rates favoring prosecutorial appeals, no clear correlation between judicial backgrounds and outcomes, and efficiency measures that balance thorough deliberation with timely resolution—though legal accuracy remains difficult to assess. The theory and new data highlight the structural idiosyncrasies of the ICC and suggest that the appellate process is serving as a reasonable internal mechanism of fairness that can support the Court’s legitimacy.