Certain areas of international law appear to imply that it is not the exclusive province of states and international and internationalized criminal courts and tribunals to conduct criminal proceedings. In particular, the law of non-international armed conflict envisages the possibility that parties to an armed conflict, including non-state organized armed groups (‘OAGs’), conduct ‘penal prosecutions’. Further, international criminal law imposes an obligation upon (commanders of) parties to an armed conflict to repress international core crimes or to submit the matter to the ‘competent authorities for investigation and prosecution’ in accordance with the doctrine of command responsibility that applies in international and non-international armed conflicts alike. That OAGs do, in fact, at least occasionally, conduct ‘criminal proceedings’ is also borne out by the facts on the ground. In a number of non-international armed conflicts, such groups have operated criminal justice mechanisms and carried out ‘investigations’ and ‘prosecutions’ and ‘convicted’ individuals for crimes, including international crimes. The questions that such ‘criminal proceedings’ raise for the complementary nature of the International Criminal Court (‘ICC’) are twofold. First, does the legal framework of complementarity in the Rome Statute accommodate proceedings carried out by OAGs inasmuch as they could potentially constitute a bar to the admissibility of cases before the ICC provided that they satisfy the requirements of ‘willingness’ and ‘ability’ that one can derive from a converse reading of Article 17? Second, would it be a wise policy choice for the Prosecutor to dismiss all ‘criminal proceedings’ by OAGs when exercising the discretion granted by Article 53 of the Statute? While it will be argued that the answer to the first question is in the negative except for the ne bis in idem scenario regulated in Article 17(1)(c) and 20(3), the second question needs careful consideration that may in certain circumstances lead to an abstention from initiating an investigation provided a number of conditions are met.