In this paper we document the prevalence of mixed immigration status families and discuss some of the immigration and citizenship policies that drive their formation. Using the 1998 Current Population Survey, we find that nearly one in ten families with children is a mixed status family: that is a family in which one or both parents is a noncitizen and one or more children is a citizen. We also find that 75 percent of children in immigrant families are citizens. We identify a number of the challenges that mixed status families pose for achieving the goals of recent welfare and illegal immigration reforms.