Highlights of findings on criminal aliens within the Federal prison system include: (1) the number of criminal aliens increased from about 42,000 at year-end 2001 to about 49,000 at year-end 2004; (2) for 2004, the majority of incarcerated criminal aliens were identified as citizens of Mexico; (3) the estimated Federal cost of incarcerating criminal aliens totaled about $5.8 billion from 2001 through 2004 ($4.2 billion direct Federal costs and $1.6 billion reimbursed to State and local governments). Highlights of findings on criminal aliens with State prisons and local jails include: (1) 5 State prison systems incarcerated about 80 percent of these criminal aliens in fiscal year 2003: Arizona, California, Florida, New York, and Texas; (2) in mid-2004, most foreign-born inmates for the 5 State prison systems with most criminal aliens were born in Mexico (60 percent); and (3) for fiscal year 2003, the State Criminal Alien Assistance Program (SCAAP) reimbursed 698 local jurisdictions for about 147,000 criminal aliens and 5 municipal county jails incarcerated about 30 percent of these criminal aliens in fiscal year 2003: Los Angeles County, CA; New York City, NY; Orange County, CA; Harris County, TX; and Maricopa County, AZ. The purpose of this study was to determine the number of criminal aliens incarcerated in Federal prisons, their country of citizenship, and to estimate the Federal cost of housing criminal aliens. The study also sought to determine the number of SCAAP criminal aliens incarcerated in State prisons and local jails and estimate the cost of incarcerating SCAAP criminal aliens in these facilities. Figures