Border security is a pivotal function in protecting the American people from terrorists and their instruments of destruction. This report uses a series of graphical presentations to form one possible framework that might assist policy makers in understanding the complex nature of border security. It is the first in a three-part series of CRS reports that make use of analytical frameworks to better understand complex problems in border security and cast them in terms that facilitate the consideration of alternative policies and practices. (CRS Report RL32840, Border and Transportation Security: Selected Programs and Policies, the second report in the series discusses selected programs and policies currently in place to address these issues. The third and final report in the series is CRS Report RL32841, Border and Transportation Security: Possible New Directions and Policy Options.) Border security is an important component of the overall homeland security effort. This effort can be seen as a series of concentric circles or screens, with the outer screen being that of preventative efforts launched outside the country-for example, interdicting or arresting terrorists and their weapons abroad before they reach the United States. The next screen is interdiction in the border security system.