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. (author abstract)