During the cold war, the US and the Soviet Union each produced enough fissile material to make tens of thousands of nuclear weapons—between 100 and 200 metric tons of plutonium and about 1000 tonnes of highly enriched uranium (uranium enriched to more than 20&percent; in U‐235). This weapons‐usable fissile material seemed invulnerable to theft until the collapse of the Soviet Union in 1991. Since then, subkilogram quantities of plutonium and multikilogram quantities of highly‐enriched uranium have been intercepted in Russia, the Czech Republic and Germany—apparently stolen from Russian nuclear facilities and intended for sale. the old Soviet security system for fissile material, which focused on the surveillance and control of those in contact with such material, has been largely swept away. Gone too is the economic security of nuclear workers, who may now be tempted or threatened by predatory criminal groups. the situation poses a grave risk to global security, because the biggest obstacle facing non‐nuclear‐weapons states or even terrorist groups interested in acquiring nuclear weapons is lack of access to the bomb material.