Recent years have witnessed an increase in the number of private military and security companies (PMCs/PSCs) operating in situations of armed conflict, as well as a change in the nature of their activities, which are now increasingly close to the heart of military operations and which often put them in close proximity to persons protected by international humanitarian law. It is often asserted that there is a vacuum in the law when it comes to their operations. In situations of armed conflict, however, there is a body of law that regulates both the activities of the staff of PMCs/PSCs and the responsibilities of the states that hire them. Moreover, other states also have a role to play in promoting respect for international humanitarian law by such companies. This article examines the key legal issues raised by PMCs/PSCs operating in situations of armed conflict, including the status of the staff of these companies and their responsibilities under international humanitarian law; the responsibilities of the states that hire them; and those of the states in whose territory PMCs/PSCs are incorporated or operate.