A Fissile Material Cutoff Treaty (FMCT) would, at minimum, end the production of highly enriched uranium (HEU) and plutonium for nuclear weapons. It is likely that the IAEA would be given responsibility for verification of an FMCT because of the overlap of its verification requirements with those of the Nonproliferation Treaty (NPT) in non-weapon states. Standard IAEA procedures and techniques for detecting undeclared enrichment and reprocessing activities and diversions from declared facilities would in many cases, however, have to be adapted for use in the weapon states. The weapon states will have stocks of pre-existing nuclear materials not subject to safeguards, pre-existing enrichment and reprocessing plants, and sensitive military nuclear facilities that would not be opened to unfettered inspection. New techniques and approaches will need to be developed to deal with these new challenges as well as with continuing weaknesses in the verification of an NPT. Specific technical challenges to be discussed include: environmental measurements near and on sites of suspected clandestine production facilities; verification of non-diversion of HEU from naval nuclear fuel cycles and of plutonium from pre-existing reprocessing plants and non-production of HEU at large enrichment plants that produced HEU in the past; and managed access arrangements, including sensors with information barriers, at military nuclear facilities.