In response to pirate attacks in the Western Indian Ocean, countries worldwide have increasingly authorized the deployment of armed guards from private military and security companies (PMSCs) on merchant ships. This widespread trend contradicts states’ commitment to retain a monopoly on violence and discourage the presence of arms on civilian vessels. This book conceptualizes the extensive use of PMSCs as a form of institutional isomorphism, combining the functionalist, ideational, political and organizational arguments used to account for the privatization of security on land into a synthetic explanation of the commercialization of vessel protection.