Presidents George W. Bush and Barack Obama have both said that the threat of nuclear terrorism is one of the nation’s greatest threats, and both presidents presided over valuable programs and initiatives that addressed many aspects of that threat. They have initiated several programs and policies to deal with the threat of nuclear terrorism, including the Proliferation Security Initiative, the Global Initiative to Combat Nuclear Terrorism, and the Nuclear Security Summits. These actions have been productive and beneficial globally, but beyond securing unguarded material and preventing nuclear or radiological terrorism, one aspect of U.S. policy against nuclear terrorism is often overlooked: deterrence. The conventional wisdom once held that terrorists are fanatics, unable to be deterred, and so devoted to their cause that they would ignore all risks. But as early as July 2002, less than a year after the 9/11 attacks, the U.S. government acknowledged that deterring terrorism was possible and had been put into practice. An unclassified CIA memo stated: “Information obtained from captured detainees has revealed that al-Qa’ida operatives are extremely security conscious and have altered their practices in response to what they have learned from the press about our capabilities.” Deterrence is the act of convincing an adversary that a course of action is too costly and unlikely to gain the intended benefits, and thus a non-event rarely will make the news. Thankfully, the United States and other countries around the world are taking proactive steps to deter nuclear terrorism, even though it may go unnoticed by the general public. For example, the Domestic Nuclear Detection Office reports that there are hundreds of radiation portal monitors located in just the sea ports around the country. Customs and Border Patrol also operates fixed and mobile nuclear detection equipment to protect thousands of miles of the U.S. land border. And it is not just federal agencies that can use equipment that could deter terrorists; even local police departments can receive grants from the Department of Homeland Security to obtain wearable radiation detection equipment. U.S. employment of radiation detection equipment for deterrence and prevention purposes introduces both uncertainty and certainty in prospective nuclear terrorists’ plans. Where and precisely how many mobile nuclear detection units are deployed in a given region creates uncertainty, yet potential attackers also come to expect that when nuclear material is detected, they would be caught and their operation would fail. As the attacks in Paris and Brussels remind us, terrorists are constantly researching and refining their attack plans based on their perceptions of their target’s security. By retaining and upgrading U.S. nuclear and radiological detection capabilities, the United States may successfully deter a terrorist nuclear attack on the homeland. Shaping terrorist adversaries’ perceptions of U.S. security is possible, and convincing them that an attempted attack with a radiological or nuclear device would fail and would have devastating consequences should remain one of America’s highest priorities.