This report hopes to fill some of the knowledge gaps in the current immigration debate by describing the unauthorized population nationally and in California and Los Angelesthe state and urban area with the largest numbers of these immigrants. Unauthorized immigrants numbered 2.45 million in California in 2004, representing almost one-quarter (24 percent) of the nation's total (10.3 million). There are about 1 million unauthorized immigrants in the Los Angeles metropolitan area, almost twice the number of any other metro area; the unauthorized are one-tenth of the area's population (10 million). The report presents findings about these populations, including their socio-economic characteristics, such as national origin, education, employment, and poverty.