Nuclear materials, especially weapons-grade, can have tremendously adverse consequences in the hands of terrorists. There needs to be a defense in depth to detect and interdict these materials, which should involve some tens of thousands of detectors world-wide. Passive detectors for this purpose have grave sensitivity problems, but these problems are often made worse by avoidable problems of the user interface. Manufacturers need to clearly understand the types of use that their equipment will be put to, the environment in which it will be located, and most especially the personnel who will be using it on a daily basis. International and national field trials have pointed out some problems with user interfaces, and these could best be resolved by manufacturers doing their own testing in simulated environments mimicing that of a customs post or other detector location.