The boundary between the United States and Mexico was created for convenient expediency through political debate and agreements (fig. 1–1). With the exception of the eastern segment of the border, which follows the course of the Rio Grande (known as the Rio Bravo in Mexico), the defining of this border was based on political decisions that had little concern for ecosystems, geologic features, or water—all of which span that imaginary line. However, the location of the border has had a remarkable effect on the biologic and physical systems in the border region and, in turn, has had a growing influence on what we now see as 21st century socioeconomic and environmental priorities. Because of the complex interactions of the human, ecological, political, and economic exigencies associated with this area, the status of the United States–Mexican border region, known as the Borderlands, has become an ever-present concern for most American citizens and for Mexican and United States Federal, State, and local governments.