This report addresses the following four questions by reporting on the impacts of Head Start on children and families during the children's preschool, kindergarten, and 1st grade years: (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? (2) What difference does Head Start make to parental practices that contribute to children's school readiness? (3) Under what circumstances does Head Start achieve the greatest impact? What works for which children? (4) What Head Start services are most related to impact? The Head Start Impact Study was conducted with a nationally representative sample of 84 grantee/delegate agencies and included nearly 5,000 newly entering, eligible 3- and 4-year-old children who were randomly assigned to either: (1) a Head Start group that had access to Head Start program services or (2) a control group that did not have access to Head Start, but could enroll in other early childhood programs or non-Head Start services selected by their parents. 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. Differential impacts are of particular interest in light of a trend of increased enrollment of the 3-year-olds in some grantee/delegate agencies presumably due to the growing availability of preschool options for 4-year-olds. Consequently, the study included two separate samples: a newly entering 3-year-old group (to be studied through two years of Head Start participation i.e., Head Start year and age 4 year, kindergarten and 1st grade), and a newly entering 4-year-old group (to be studied through one year of Head Start participation, kindergarten and 1st grade). The study showed that the two age cohorts varied in demographic characteristics, making it even more appropriate to examine them separately. 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 (Black children 32.8%, Hispanic children 37.4%, and White/other children 29.8%), about half of newly entering 4-year-olds were Hispanic children (Black children 17.5%, Hispanic children 51.6%, and White/other children 30.8%). The ethnic difference is also reflected in the age-group differences in child and parent language. This report presents the findings from the preschool years through children's 1st grade experience. This document consists of the Executive Summary and nine chapters. Chapter 1 presents the study background, including a literature review of related Head Start research and the study purpose and objectives. Chapter 2 provides details about the study design and implementation. It discusses the experimental design, sample selection prior to random assignment, data collection, and data analysis. To provide a context in which to understand the impact findings, Chapter 3 examines the impact of Head Start on the services and child care settings that children experience prior to starting school. It also provides the impact of Head Start on the educational and child care settings, setting characteristics, and services that children experience during kindergarten and 1st grade. Chapters 4 through 7 present the impact of Head Start on children's outcomes and parenting practices for the years before school and then for kindergarten and 1st grade. Chapter 4 presents the impact of Head Start on children's cognitive development, Chapter 5 presents the impact of Head Start on children's social-emotional development, Chapter 6 presents the impact of Head Start on children's health status and access to health services, and Chapter 7 presents the impact of Head Start on parenting practices in the areas of educational activities, discipline practices, and school involvement. Chapter 8 examines variation in impacts by child characteristics, parent and family characteristics, and community characteristics. Chapter 9 provides an overall summary of the findings, implications for the Head Start Program, and unanswered questions. Appendices in this volume include the Head Start Impact Study legislation, a list of the official Head Start Impact Study Advisory Committee members, the language decision form used to determine the language in which the child was assessed, and data tables that elaborate on the findings presented in the volume (e.g., Impact on Treated (IOT) findings). The findings from a sample of programs in Puerto Rico are also provided in an appendix. Programs in Puerto Rico were included in the study with the intent that data on children in these programs would be analyzed along with the data on children in the 50 states and the District of Columbia, once children reached school-age. (Contains 1 figure, 117 footnotes, and 114 exhibits.) [The ERIC version of this document contains the following supplementary materials: Head Start Impact Study Main Impact Tables, 2003 through 2006; and Head Start Impact Study Subgroup Impact Tables, 2003 through 2006. For the "Head Start Impact Study Technical Report," see ED507846. For the "Head Start Impact Study Final Report. Executive Summary," see ED507847.]