Frequently characterized as either mercenaries in modern guise or the market's response to a security vacuum, private military companies are commercial firms offering military services ranging from combat and military training and advice to logistical support, and play an increasingly important role in armed conflicts, UN peace operations, and providing security in unstable states. As private military companies assume more responsibilities in conflict and post-conflict settings, their growing significance raises fundamental questions about their nature, their role in different regions and contexts, and their regulation. This volume examines the interaction between regulation and market forces and analyzes the current legal framework and the needs and possibilities for regulation in the years ahead.