In fiscal year 2006, the law enforcement and criminal justice costs associated with undocumented immigration to these 24 border counties were $192 million. Over the past 8 fiscal years, from 1999 through 2006, the costs have added up to a staggering $1.23 billion. A fundamental principle of immigration law since 1790 is that the Federal Government has primary power and responsibility, related to several Constitutional provisions. However, the Federal Government has only reimbursed these 24 counties $4.7 million for detaining criminal undocumented immigrants for fiscal year 2006 (and $54.8 million since 1999). Many members of Congress have recognized that costs to counties associated with undocumented immigration are a Federal responsibility and reimbursement is the only appropriate action when costs are documented. This study supported by the U.S. Department of Justice, National Institute of Justice researched the law enforcement and criminal justice agencies in all 24 counties within the States of California (San Diego and Imperial), Arizona (Yuma, Pima, Santa Cruz, and Cochise), New Mexico (Hidalgo, Luna, and Dona Ana), and Texas (El Paso, Hudspeth, Culberson, Jeff Davis, Presidio, Brewster, Terrell, Val Verde, Maverick, Kinney, Webb, Zapata, Starr, Hidalgo, and Cameron); fiscal data were compiled for 1 fiscal year, 2006, on expenditures from the county general fund. This report is about the costs to county governments of providing services for populations that are the responsibility of the Federal Government. The costs of the Federal Government’s failed immigration policy disproportionately substantially affect these 24 counties along the United States-Mexico border. Tables