Within the Department of Homeland Security's (DHS's) Customs and Border Protection (CBP), the U.S. Border Patrol (USBP) is charged with securing our nation's land and maritime borders between official ports of entry (POE) to deter and interdict terrorists, weapons of mass destruction, and aliens attempting to enter the country unlawfully. In order to discharge its duties, the USBP deploys personnel, technology, and tactical infrastructure such as vehicle barriers and fencing. Fencing is erected on the border to impede the illegal entry of unauthorized aliens, while vehicle barriers are designed to impede the entry of vehicles but do not impede the entry of individuals. This report will analyze the barriers that are currently being constructed and maintained along the border by the USBP, including historical and future cost estimates and the policy issues involved. Because the current debate has largely focused on the deployment of fencing to the border, this report will focus on the policy issues surrounding the construction of border fencing. However, information concerning the kinds of vehicle barriers being deployed at the border will be provided where available. Using the broad powers granted to the Attorney General (AG) to control and guard the U.S. border, the USBP began erecting a barrier known as the "primary fence" directly on the border in 1990 to deter illegal entries and drug smuggling in its San Diego sector. The San Diego fence formed part of the USBP's "Prevention Through Deterrence" strategy, which called for reducing unauthorized migration by placing agents and resources directly on the border along population centers in order to deter would-be migrants from entering the country. The San Diego primary fence was completed in 1993, covering the first 14 miles of the border from the Pacific Ocean. The fence was constructed of 10-foot-high welded steel army surplus landing mats with the assistance of the Corps of Engineers and the California National Guard. In addition to the 14 miles of primary fencing erected in its San Diego sector, the USBP maintains stretches of primary fencing in several other sectors along the southwest border, including Campo, CA; Yuma, AZ; Nogales, AZ; Naco, AZ; Douglas, AZ; and El Paso, TX. In 1996, Congress passed the Illegal Immigration Reform and Immigrant Responsibility Act (IIRIRA), which, among other things, explicitly gave the Attorney General broad authority to construct barriers along the border and authorized the Immigration and Naturalization Service (INS) to construct a secondary layer of fencing to buttress the completed 14-mile primary fence. Construction of the secondary fence stalled after 9.5 miles had been completed due to environmental concerns raised by the California Coastal Commission (CCC). In 2005, Congress passed the REAL ID Act, which, among other things, authorized the Secretary of the Department of Homeland Security (DHS) to waive all legal requirements to expedite the construction of border barriers. In 2006, Congress passed the Secure Fence Act, which, among other things, directed DHS to construct five separate stretches of fencing along the southern border, totaling 850 miles. This requirement was modified by provisions in Division E of H.R. 2764 , the Consolidated Appropriations Act, 2008 ( P.L. 110-161 ), which was enacted into law on December 26, 2007. The Secretary of Homeland Security is now required to construct reinforced fencing along not fewer than 700 miles of the southwest border, in locations where fencing is deemed most practical and effective. In addition to border fencing, the USBP deploys both permanent and temporary vehicle barriers at the border. Vehicle barriers are meant to stop the entry of vehicles, but not people, into the United States. Temporary vehicle barriers are typically chained together and can be moved to different locations at the USBP's discretion. Permanent vehicle barriers are embedded in the ground and are meant to remain in one location. The USBP's San Diego sector extends along the first 66 miles from the Pacific Ocean of the international border with Mexico, and covers approximately 7,000 square miles of territory. Located north of Tijuana and Tecate, Mexican cities with a combined population of more than two million people, the sector features no natural barriers to entry by unauthorized migrants and smugglers. As a result of this geographical reality and in response to the large numbers of unauthorized aliens crossing the border in the area, in 1990 the USBP began erecting a physical barrier to deter illegal entries and drug smuggling. The ensuing "primary" fence covered the first 14 miles of the border, starting from the Pacific Ocean, and was constructed of 10-foot-high welded steel. The primary fence, by itself, did not have a discernible impact on the influx of unauthorized aliens coming across the border in San Diego. As a result of this, Operation Gatekeeper was officially announced in the San Diego sector on October 1, 1994. The chief elements of the operation were large increases in the overall manpower of the sector, and the deployment of USBP personnel directly along the border to deter illegal entry. The strategic plan called for three tiers of agent deployment. The first tier of agents was deployed to fixed positions on the border. The agents in this first tier were charged with preventing illegal entry, apprehending those who attempted to enter, and generally observing the border. A second tier of agents was deployed north of the border in the corridors that were heavily used by illegal aliens. The second tier of agents had more freedom of movement than the first tier and were charged with containing and apprehending those aliens who made it past the first tier. The third tier of agents were typically assigned to man vehicle checkpoints further inland to apprehend the traffic that eluded the first two tiers. As the Department of Justice Inspector General report notes, "given Gatekeeper's deterrence emphasis, many agents were assigned to first-tier, fixed positions along the border. These agents were instructed to remain in their assigned positions rather than chase alien traffic passing through adjacent areas. Prior to Gatekeeper, such stationary positions were relatively rare." Operation Gatekeeper resulted in significant increases in the manpower and other resources deployed to San Diego sector. Agents received additional night vision goggles, portable radios, and four-wheel drive vehicles, and light towers and seismic sensors were deployed. According to the former INS, between October 1994 and June of 1998, San Diego sector saw the following increases in resources: USBP agent manpower increased by 150%; Seismic sensors deployed increased by 171%; Vehicle fleet increased by 152%. Infrared night-vision goggles increased from 12 to 49; Permanent lighting increased from 1 mile to 6 miles, and 100 portable lighting platforms were deployed; Helicopter fleet increased from 6 to 10. As a result of the increase in resources and the new strategy that were the main components of Operation Gatekeeper, the USBP estimated in 1998 that the entire 66 miles of border patrolled by the San Diego sector's agents could be brought under control in five years. According to CBP, the primary fence, in combination with various USBP enforcement initiatives along the San Diego border region (i.e., Operation Gatekeeper), proved to be successful but fiscally and environmentally costly. For example, as unauthorized aliens and smugglers breached the primary fence and attempted to evade detection, USBP agents were often forced to pursue the suspects through environmentally sensitive areas. It soon became apparent to immigration officials and lawmakers that the USBP needed, among other things, a "rigid" enforcement system that could integrate infrastructure (i.e., a multi-tiered fence and roads), manpower, and new technologies to further control the border region. The concept of a three-tiered fence system was first recommended by a 1993 Sandia Laboratory study commissioned by the former Immigration and Naturalization Service (INS). According to the Sandia study, the use of multiple barriers in urban areas would increase the USBP's ability to discourage a significant number of illegal border crossers, to detect intruders early and delay them as long as possible, and to channel a reduced number of illegal border crossers to geographic locations where the USBP was better prepared to deal with them. The Sandia study further noted that segments of the border could not be controlled at the immediate border due to the ruggedness of the terrain, and recommended the use of highway checkpoints in those areas to contain aliens after they had entered the country illegally. The study concluded that aliens attempting to enter the United States from Mexico had shown remarkable resiliency in bypassing or destroying obstacles in their path, including the existing primary fence, and postulated that "[a] three-fence barrier system with vehicle patrol roads between the fences and lights will provide the necessary discouragement." As previously mentioned, the INS constructed the primary fencing in San Diego using the broad authority granted to the AG in order to guard and control the U.S. border by the Immigration and Nationality Act (INA). In 1996, Congress expressly authorized the AG to construct barriers at the border for the first time in the Illegal Immigration Reform and Immigrant Responsibility Act (IIRIRA). This legislation has subsequently been amended on several occasions. Section 102 of IIRIRA concerned the improvement and construction of barriers at our international borders. As originally enacted, § 102(a) appeared to give the AG broad authority to install additional physical barriers and roads "in the vicinity of the United States border to deter illegal crossings in areas of high illegal entry into the United States." The phrase "vicinity of the United States border" was not defined in the INA or in immigration regulations. The section also did not stipulate what specific characteristics would designate an area as one of "high illegal entry." As originally enacted, § 102(b) mandated that the AG construct a barrier in the border area near San Diego.