While early private military and security companies (PMSCs) were likened to mercenaries, today most scholars agree that PMSCs constitute a new phenomenon. They are organized as legitimate corporate entities, have a distinct legal status, and provide a wide range of military and security services. This definition reflects the evolution of the PMSC industry, which has moved beyond combat services to supply everything from transport, logistics, and maintenance to military and police training, demining, intelligence, risk analysis, armed and unarmed protective services, anti-piracy measures, border protection, and drone operations. Not only have PMSC services diversified, but so has their client base. In addition to industrialized and failed states, transnational corporations, international organizations, and even NGOs increasingly make use of PMSCs. There are several explanations for the growing recourse to these companies. Functional explanations see the employment of PMSCs as a rational response to the glaring gap between demand and supply in the market for force. Ideational and constructivist approaches, by contrast, impute national differences in the outsourcing of military and security services to dominant beliefs and norms about the appropriate relationship between the state and the market. The consequences of using PMSCs, including the accountability, effectiveness, and state control of PMSCs, issues of gender and racial equality, and theoretical implications for the location of political authority and the public good character of security are key issues. So is the question of suitable forms of regulation for the industry, including national and international laws, informal industry self-regulations, and hybrid regulatory approaches such as multi-stakeholder initiatives and standard setting schemes.