The recent wars in Iraq and Afghanistan have been characterized by the deployment of large private military forces, under contract with the US administration. The use of so-called private military corporations (PMCs) and, more generally, of mercenaries, has long attracted criticisms. This article argues that under certain conditions (drawn from the Just War tradition), there is nothing inherently objectionable about mercenarism. It begins by exposing a weakness in the most obvious justification for mercenarism, to wit, the justification from freedom of occupational choice. It then deploys a less obvious, but stronger, argument - one that appeals to the importance of enabling just defensive killings. Finally, it rebuts five moral objections to mercenarism.