This article examines the ethics of using private security companies to undertake combat operations in modern conflict zones. Previous studies on this topic, including those that have drawn on the principles of just war theory, have, out of necessity, been highly speculative because they lacked a strong empirical basis on which to evaluate the behaviour of private security personnel during their operations. Indeed, most scholarship on the ethics of private security companies has relied on a handful of anecdotal examples that happened to receive extensive media coverage. In contrast, this article undertakes the first quantitative analysis of how well the employees of a dozen private security companies adhered to the jus in bello tenets of just war theory and also how their degree of adherence to these tenets affected their tendency to suffer friendly casualties during their security operations in Iraq. It finds that the employees of most of the firms under study exhibited a moderate or high level of adherence to the jus in bello principles of proportionality and discrimination during their security operations in Iraq. Moreover, it also finds that close adherence to these principles did not necessarily expose private security personnel to greater risk of suffering harm.