Article 51(3) of Additional Protocol I (for international armed conflicts) and Article 13(3) of Additional Protocol II (for non-international armed conflicts) articulate the limits to the scope of protected civilian status: 'Civilians shall enjoy the protection afforded by this Section, unless and for such time as they take a direct part in hostilities.' Civilians who participate in hostilities are often referred to by the accronym DPH. Defining DPH has proven to be problematic. The meaning of these words set out in Article 51(3) has ignited intense debate. A major development in this area of the law occured with the publication of the ICRC Interpretative Guidance on Direct Participation in Hostilities in 2009.The arguments of this chapter engage with those of the Guidance, the most comprehensive legal document on the meaning of 'civilians directly participating in hostilities'.