Failing and collapsed states are a common marketplace for the private military industry, which has grown significantly in size and scope over the last decade. Today the private sector supplies a broad spectrum of military and security services to governments facing a lack of territorial control and law enforcement capacities. These services range from combat support to training for military and policing units, logistics and the protection of individuals and property. Yet a quantifiable picture of the extent to which these private security services are being used by failing or weak governments and the implications this use might have for the security environment has not been properly painted. This paper aims to fill this gap by presenting statistical findings on the use of private military and security companies (PMSCs) in failing states. It utilises data from the Private Security Database that account for instances of military outsourcing by public actors (governments and international organisations) in failing states in the period 1990–2007. Starting from the assumption that PMSCs play an increasingly important role in the security environment in failing states by supplementing, substituting or compensating public forces, the paper raises three interlinked questions. To what extend is private security a common feature in countries that face episodes of state failure? Under which conditions are PMSCs present in countries with weak or failinggovernments? And what kind of effect do PMSCs have on political instability in general? To address these research questions, the paper reviews the literature on the strategic role of PMSCs in contexts of conflict and state failure, and deduces empirically testable propositions and expectations based on the perspective of advocates and critics – arguments for and against. The data analysis is structured to reflect views from both sides. The key findings suggest that PMSCs are in fact increasingly involved in countries that face episodes of state failure. They tend to enter the theatre during and after those episodes – but do not trigger their outbreak – and accompany foreign intervention forces rather than acting on their own behalf. This finding is supported by the fact that there has been a significant shift towards an external client base since the end of the 1990s. When present in these countries, generally speaking PMSCs do not significantly contribute to a shortening of episodes of political instability; on the other hand, they do not harm local security institutions in terms of resource draining or militarisation. By presenting these additional quantitative insights, this paper contributes to the evaluation of theoretical arguments made about conditions and effects of private provision of security in countries where the public sector is limited in its ability to enforce the monopoly of violence and implement collectively binding decisions.