Book excerpt: Border enforcement is intended to prevent and deter the illegal movement of goods and people across a country's border. The intensification of border enforcement activities creates impediments to illegal entry that increase the costs incurred by migrants when crossing the border. Increased costs may include additional time investment, physical hardship, and higher fees charged by smugglers who assist migrants across the border. The impact of enforcement on illegal immigration depends on how enforcement affects migration costs and how migration costs affect the decision to migrate. Our goal in this paper is to estimate the impact that enforcement has on the price smugglers charge to bring illegal immigrants across the U.S.-Mexico border. The degree to which migration costs rise in response to intensified enforcement can be termed the cost elasticity with respect to enforcement, and the degree to which the number of potential illegal immigrants falls in response to increased costs can be termed the migration elasticity with respect to cost. One important aspect of migration costs that can be quantified and measured is the fee charged by smugglers of illegal immigrants. Intensified enforcement activities should increase the difficulty and cost to a smuggler for getting clients successfully across the border, and this would be passed on to clients through an increase in the smuggling cost. It is probable that an increased smuggling cost would deter potential illegal immigration, thus creating a level of deterrence. The degree to which border enforcement activities have prevented and deterred the movement of illegal immigrants across the Southwest border of the United States is an issue of public policy importance and has been the subject of considerable debate. It is also a challenging measurement issue with respect to both data availability and statistical methodology. In this study, we estimate the value of the smuggling cost elasticity with respect to enforcement on the Southwest border of the United States using data on the price charged by smugglers to assist illegal immigration, as collected by the United States Border Patrol (USBP) from individuals apprehended at the border, and the level of enforcement activity, as captured by hours spent on enforcement activity by the USBP. Apprehended migrants respond to a series of questions by Border Patrol agents, including whether they were smuggled and how much they paid the smuggler. The data are then reported as part of an administrative apprehension record. Collection of such records began in certain sectors in the early 1990s and was comprehensively implemented in all sectors along the Southwest border by fiscal year 1999. Using data collected along the entire Southwest border on smuggling cost and enforcement hours at the apprehended migrant's crossing location at the monthly frequency, we estimate the smuggling cost elasticity with respect to enforcement.