International efforts to improve the security of nuclear material and facilities from theft and sabotage were intensified by the attacks of September 11. But the magnitude of the changes that are needed to protect against terrorist attacks like those of September 11 has not yet been widely appreciated, perhaps in part because of beliefs in some countries that what happened in the US on September 11 “can’t happen here.” This report will discuss the relevant features of the September 11 attacks, the recent illicit trafficking in nuclear and radioactive materials that might benefit terrorists, and the international efforts to improve physical protection after September 11.