Since the International Criminal Court’s inception to date, most of the litigation before the Appeals Chamber on the various aspects of the Court’s statutory framework has addressed procedural and evidentiary matters. Only a few have been concerned with substantive law. This chapter focuses on the Appeals Chamber’s jurisprudence on the crimes and modes of liability where the Chamber has had an opportunity to pronounce itself on those issues such as the delineation and requirements of articles 8(2)(b)(xxvi) and 8(2)(e)(vii) of the Rome Statute on conscripting, enlisting or using children under the age of fifteen years in international and non-international armed conflicts respectively, and modes of criminal responsibility such as co-perpetration under article 25 (3)(a) and common purpose liability under article 25(3)(d). In addition, it addresses how the Appeals Chamber resolves the question of whether war crimes can be committed against members of the same armed force or group under article 8 of the Rome Statute.