International humanitarian law places certain obligations on, or gives rights to, a “party to the conflict” or “parties to the conflict”. Although nearly 200 provisions of the Geneva Conventions and their Additional Protocols include such a phrase, none of these instruments defines this term. At the same time, even though terrorist groups or organisations may be using armed force during armed conflicts, States appear reluctant to recognise such groups as parties to a conflict. The present contribution explains what is to be understood as parties to the conflict for each of the two types of armed conflict under international humanitarian law, namely international armed conflicts and non-international armed conflicts, and discusses whether, or to what extent, (alleged) terrorist organisations can qualify as such parties. This contribution further critically assesses the findings in recent Belgian case law that various armed actors involved in the Syrian conflict are not considered to be parties to the said conflict.