The evaluation of Early Head Start was carried out in 17 selected local programs across the country that agreed to randomly assign eligible families to either the program or to a comparison group. Sites were selected to ensure geographic and program diversity. Center-based, home-based, and mixed approaches to service delivery were examined. Data sources included periodic parent interviews; field interviewers’ child and family assessments; Head Start Family Information System data; site visit observations; and program staff ratings of families’ engagement. Implementation data were collected in three rounds of site visits. A panel assessed the degree of implementation independently and for different areas.