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. Data collection began in fall 2002 and continued through 2006, following children from program application through the spring of their 1st grade year. 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. (author abstract)