"Unauthorized Migrants: Numbers and Characteristics" was prepared by Jeffrey S. Passel, a veteran demographer and senior research associate at the Center, using a well-established methodology to analyze data from the March 2004 Current Population Survey, which was conducted by the Census Bureau and the Bureau of Labor Statistics. The report builds on previous work that estimated the size and geographic dispersal of the undocumented population and offers a portrait of that population in unprecedented detail by examining family composition, educational attainment, income and employment. The report estimates the number of persons living in families in which the head of the household or the spouse is an unauthorized migrant--13.9 million as of March 2004, including 4.7 million children. Of those individuals, some 3.2 million are US citizens by birth but are living in "mixed status" families in which some members are unauthorized, usually a parent, while others, usually children, are Americans by birthright. Extensive data on the employment of unauthorized migrants maps their presence in many sectors of the US labor force.