The United States increasingly relies on unmanned aerial vehicles-better known as drones-to target insurgent and terrorist groups around the world. Drones have been used in armed conflicts in which the United States is a recognized participant, including the conflicts against insurgent groups in Afghanistan and Iraq, and against government forces in Libya. The United States has also used drones to strike at terrorist and insurgent groups outside of theaters of armed conflict. These include drone strikes that target militants in Pakistan who support al-Qaeda and insurgents operating in Afghanistan, al-Qaeda in the Arabian Peninsula in Yemen, and the al-Shabaab movement in Somalia. The objectives of these campaigns of drone strikes are to punish and to deter insurgent and terrorist organizations. They punish these organizations by killing and creating fear and uncertainty among current members. They also seek to deter insurgents and terrorists from engaging in more violence, as well as to deter others from joining or supporting these movements. While drones have attracted considerable attention, we know little about how effective they are as tools of punishment and deterrence. In particular, it is not clear how, if at all, drones differ from other technologies of violence, what experience with broadly similar technologies in past conflicts suggests will be the likely consequences of drone strikes, and what systematic analysis of the available evidence suggests about the effects of the drone campaigns.