In the Just War tradition, it has been the norm that the use of mercenaries violated any claim to a just war. Just War Theorists have argued that since states employing mercenaries cannot satisfy all of the war decision principles, any state which employs mercenaries in warfare is conducting an unjust war. This line of reasoning should also been applied to contemporary private military companies (PMCs) according to many Just War theorists. In this thesis, I argue against the prohibition on PMCs in Just War Theory. PMCs can be employed in certain combat scenarios while satisfying Just War decision principles. Specifically, there is no principled reason to refrain from employing PMCs in humanitarian intervention. The use of PMCs can fill a niche left open by stronger states and the international community in preventing mass violations of human rights in places such as Rwanda in 1994.