The rise of Private Military Companies (PMCs) and Private Security Companies (PSCs) in military or civilian missions stands as an important change in the dynamics of the security and defence world, as they change how operations can be conducted, especially for armed forces. Dating back to the Middle Ages with mercenaries, they have taken different forms throughout history. Nowadays, they deliver a large span of services (from indirect support in logistics or military counselling to taking part directly in combat). This paper will present their blurred status in law and their duties in missions led by European Member States, taking a look at the existing legislation and tools to hold them responsible for their actions.