The use of scanning systems (such as radiation portal monitors and X-ray imagers) at border checkpoints is said to act as a deterrent to the smuggling of radiological/nuclear materials, drugs, and other illicit items. Can such deterrent effects be measured? The author examines several mathematical formulations of the deterrence function—the probability that a contemplated act of smuggling will be carried out, given that scanning occurs at a given rate and level of effectiveness. These formulations are further developed as “extended” models that can incorporate the different motivations, perceptions, and decision-making behaviors of different smuggling populations. Extended model predictions regarding deterrence thresholds are compared to individual cases of radiological/nuclear smuggling and aggregated data on drug smuggling activity. These comparisons point to some tentative conclusions regarding the conditions under which scanning systems might or might not act as a deterrent. Such conclusions are necessarily limited by the study’s reliance on open-source information, the relatively small number of cross-border radiological/nuclear smuggling cases on record, the high level of aggregation in the drug smuggling data used, the need to make very rough estimates of some intermediate variables, and the fact that cognitive and behavioral models (including models of decision-making under conditions of risk and uncertainty) have not been validated for the specific populations being studied here.