The Congressionally-mandated Head Start Impact Study is being conducted across 84 nationally representative grantee/delegate agencies. Approximately 5,000 newly entering 3- and 4-year-old children applying for Head Start were randomly assigned to either a Head Start group that had access to Head Start program services or to a non-Head Start group that could enroll in available community non-Head Start services, selected by their parents. Data collection began in fall 2002 and is scheduled to continue through 2006, following children through the spring of their 1st-grade year. The study quantifies the impact of Head Start separately for 3- and 4-year-old children across child cognitive, social-emotional, and health domains as well as on parenting practices. For children in the 3-year-old group, the preliminary results from the first year of data collection demonstrate small to moderate positive effects favoring the children enrolled in Head Start for some outcomes in each domain. Fewer positive impacts were found for children in the 4-year-old group. Appended are: (1) Section 649(G) of the Head Start Act, 1998 (PL 105-285); (2) Calculating Analytical Sampling Weights for Fall 2002 and Spring 2003; (3) Language Decision Form; (4) Citations for Child Assessments, Scales, and Observation Instruments; (5) Comparison of Head Start Grantees/Delegate Agencies and Centers in Saturated and Non-Saturated Communities; (6) Determination of Head Start Participation; (7) The Racial/Ethnic Composition of the Study Sample; (8) Differences between Main Arrangement and Focal Arrangement; (9) Imputations for Item Nonresponse in the Fall 2002 Data; (10) Comparison of Weighted and Unweighted Mean Differences by Age Cohort; (11) Impact Regression Procedures; (12) Measures Of Fall 2002 "Starting Points" Used in the Regression Models, by Child and Parent Outcomes; (13) Tests for Lack of Impact of Head Start on Demographic and Developmental Factors Measured in Fall 2002; (14) Basis for Assuming That Non-Participants Experienced No Intervention Effects; (15) Cognitive Domain, Estimated Impact on Program Participants; (16) Factors That Moderate the Impact of Head Start: Detailed Tables for Cognitive Outcomes; (17) Social-Emotional Domain Estimated Impact on Program Participants; (18) Factors That Moderate the Impact of Head Start: Detailed Tables for Social-Emotional Outcomes; (19) Health Domain, Estimated Impact of Program Participation; (20) Factors That Moderate the Impact of Head Start: Detailed Tables for Health Outcomes; (21) Parenting Practices Domain, Estimated Impact of Program Participation; and (22) Factors That Moderate the Impact of Head Start: Detailed Tables for Parenting Outcomes. Individual sections contain footnotes. (Contains 47 exhibits.) [Contributors include: Nicholas Zill, Gary Shapiro, Pam Broene, Debra Mekos, Monica Rohacek, Liz Quinn, Gina Adams, Janet Friedman, and Haidee Bernstein. For the "Head Start Impact Study: First Year Findings. Executive Summary," see ED543009.]