The U.S. Department of Homeland Security is responsible for controlling the flow of goods and people across the U.S. border, a difficult task that raises challenging resource management questions about how best to minimize illicit flows across the border while facilitating legitimate ones. Commonly reported border control measures, such as numbers of illegal migrants apprehended or miles of border under effective control, bear only an indirect and uncertain relationship to the border control mission, making them unreliable management tools. Fundamental to the question of border control effectiveness is the proportion of illicit border crossings that are prevented through either deterrence or apprehension. Estimating these proportions requires knowing the total flow of illicit goods or border crossings, but compelling methods for producing such estimates do not yet exist. This short paper describes four innovative approaches to estimating the total flow of illicit border crossings between ports of entry. Each is sufficiently promising to warrant further attention for purposes of supporting reliable, valid, and timely measures of illicit cross-border flow. Successfully implementing each of these approaches will require methodological development and analysis to identify barriers or constraints to using the approach, the cost of data collection, and the amount of error that can be expected in the resulting estimates.