In the 1998 reauthorization of Head Start, Congress mandated that the US Department of Health and Human Services (DHHS) determine, on a national level, the impact of Head Start on the children it serves. As noted by the Advisory Committee on Head Start Research, this legislative mandate required that the impact study address two main research questions: (1) "What difference does Head Start make to key outcomes of development and learning (and in particular, the multiple domains of school readiness) for low-income children? What difference does Head Start make to parental practices that contribute to children's school readiness?"; and (2) "Under what circumstances does Head Start achieve the greatest impact? What works for which children? What Head Start services are most related to impact?" The "Head Start Impact Study Final Report" addressed these questions by reporting on the impacts of Head Start on children and families during the children's preschool, kindergarten, and 1st grade years. This "Third Grade Follow-up to the Head Start Impact Study Final Report" addresses these same questions by looking at longer-term effects through the end of 3rd grade. The study was designed to separately examine two cohorts of children, newly entering 3-and 4-year-olds. This design reflects the hypothesis that different program impacts may be associated with different age of entry into Head Start. The study showed that the two age cohorts varied in demographic characteristics. The racial/ethnic characteristics of newly entering children in the 3-year-old cohort were substantially different from the characteristics of children in the newly entering 4-year-old cohort. While the newly entering 3-year-olds were relatively evenly distributed between Black children and Hispanic children about half of newly entering 4-year-olds were Hispanic children. The ethnic difference was also reflected in the age-group differences in child and parent language. Appended are: (1) Updated Weighting and Analysis Procedures; (2) Baseline Characteristics for the 3rd Grade Analysis Sample; (3) Intent to Treat (ITT) Tables, 2002-2008; (4) Impact on the Treated (IOT) Tables, 2003-2008; and (5) Subgroup Tables, 2003-2008. (Contains 104 exhibits and 85 footnotes.) [For "Third Grade Follow-Up to the Head Start Impact Study: Final Report. OPRE Report 2012-45b. Executive Summary," see ED539263.