The idea behind the Nuclear Security Summits was to prevent terrorist groups such as the Islamic State from gaining access to nuclear weapons, fissile materials, and nuclear facilities. But nuclear security is never “done”—not as long as fissile and radiological materials exist—so even now, with the summit process complete, the threat of nuclear terrorism is not necessarily diminishing. Truth is, the risk of nuclear terrorism cannot be eliminated. But if states make the utmost commitment to protecting nuclear materials, nuclear security can be continuously improved. The 2016 summit in Washington eased the way toward continuous improvement by establishing a set of nuclear security mechanisms that all states can join and all can implement—even without summits.