In response to reports of frequent criminal misconduct, aggressive behavior, and human rights abuses committed with impunity by private contractors in Iraq and Afghanistan, some have argued that private military and security companies (“PMSCs”) are no more than modern mercenaries, and that they should therefore be banned under the standing international prohibition on mercenarism. However, the existing instruments prohibiting mercenarism would be difficult to apply to most PMSCs, making it easy for states that want to continue to use these companies to evade such a ban. In contrast, given market forces pushing PMSCs to be more compliant and emerging state practices that favor regulation, coordinated international regulation of PMSCs might feasibly be enforced. This article proposes that many of the issues with private military and security companies could be addressed by creating an international humanitarian law (“IHL”) principle that recognizes state use of PMSCs as a means of warfare. The availability of advanced, independent security and military capabilities-for-hire enables states or nonstate actors to get around political or resource constraints that otherwise might limit the use of force, and may undermine IHL enforcement. These threats might be addressed if IHL established a stronger state responsibility link between states and the PMSCs they hire. International humanitarian law should provide that states who outsource government security or military functions in support of any combat or humanitarian operations that would otherwise trigger IHL must establish internal oversight, accountability, and liability mechanisms to ensure that these actors comply with international and domestic legal norms and regulations.