Unmanned combat air vehicles, or in common parlance 'drones', have become a prominent instrument in US efforts to counter an objective (and subjective) cross-border terrorist threat with lethal force. As a result, critical questions abound on the legitimacy of their use. In a series of multidisciplinary essays by scholars with an extensive knowledge of international norms, this book explores the question of legitimacy through the conceptual lenses of legality, morality and efficacy, it then closes with the consideration of a policy proposal aimed at incorporating all three indispensable elements.