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 shows how they adapted to the most modern types of conflict thanks to their business structure and flexibility in various scenarios. On the one hand, PMCs participate in combats, from troops through training or instruction programmes, having a strong impact on the development of conflicts. On the other hand, PSCs dedicate themselves to tasks related to security and guard duties like protection of facilities or personnel in at- risk zones. While they both provide services to governments, PMCs are employed in training military forces and taking part in conflict zones, and PSCs are used primarily for non-combat missions such as personnel protection abroad and site security. However, by the nature of their work, both PMCs and PSCs may be engaged in violent scenarios. Therefore, it is also generally accepted to refer to them as Private Military and Security companies (PMSCs). The employment of additional resources to the national armies is a phenomenon which requires a comprehensive regulation on a national and international level. It is necessary, however, to draw a line between PMCs and PSCs, even if the lack of regulation and the similarity of tasks unite them. If the classification of PMSCs personnel in concrete category under the Geneva Conventions seems complex, the distinction between soldiers and civilians nevertheless still has to be made to understand what they can and cannot do in conflicts, eventually having an incidence on the rules of engagement they have to follow and the protection they could be granted or not. 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. Analysing the relevant dispositions of International Law, this paper will try to clarify the risks and potential downfalls of employing such companies for domains normally part of State responsibilities solely. Furthermore, this paper will analyse the complex and fragmented legal framework governing these companies and their employment both on a European and national level, delving into the distinction of competencies between the EU and the national sovereignty of Member States, according to their own regulation on the matter. This will also be done through practical studies of the use of PMSCs in the Balkans region in the 1990s and in the Afghan conflict in the 21st century. Moreover, the paper will tackle the urgent necessity to strengthen the rules and norms that limit the actions of PMSCs to make sure that human rights and ethical boundaries are respected while they pursue their contracts.