In this study, the authors used data from a large longitudinal birth cohort study of primarily low-income children in urban areas, the Fragile Families and Child Wellbeing Study (FFCWS), to investigate the effects of Head Start participation on children's school readiness. The fact that their sample was mainly made up of disadvantaged families helped address some of the issues with regard to selection bias, but to further address possible selection bias, they adopted several different analytic approaches, including ordinary least squares (OLS) regressions with a rich set of controls, city-fixed effects, and propensity score matching models. In addition, they were able to control for children's earlier developmental outcomes (i.e., at age three when almost none of the children had attended Head Start), which has not been possible in most of the previous research. In common with prior studies, they first examined the effects of Head Start by comparing Head Start participants to all non-participants (regardless of what their child care arrangements were). Then, to address the problem of the lack of clarity with regard to the reference group, they compared children who attended Head Start to children who attended specific types of other care arrangements separately, including parental care, pre-kindergarten, other center-based care, and other non-parental care. They also analyzed whether the effects of Head Start were moderated by child gender and race/ethnicity. (Contains 2 tables.)