Recent decades have seen an increasing reliance on private military contractors (PMCs) to provide logistical services, training, maintenance, and combat troops. By contrast, just war theorizing is generally built on the assumption that states, and states alone, wield a monopoly on the legitimate use of force and participate in war. The mismatch between these assumptions and the increasing privatization of war pose some important problems. Who holds responsibility for the actions of PMCs? What ethical standards might they be required to observe? How can we know if a war fought by private actors is just?