Along with the rapid expansion over the past decade of Head Start, a program providing comprehensive early childhood development services to low-income children, their families, and their communities, has come the demand for rigorous research to demonstrate program effectiveness. This report describes the proposed design of a national study of the impact of Head Start on children, mandated through the 1998 reauthorization of Head Start. The study will entail the selection of a sample of Head Start applicants who will be randomly assigned to either a treatment or a control group. A field test is proposed to run parallel with initial site recruitment to inform later design modifications or refinements. The focus of the study will be the impact of federal Head Start on children, how program effects vary along a variety of dimensions, and the contribution of the Head Start service model to child outcomes compared to those of less intensive services. Other information included in the report are details of the research goals and objectives; the selection of study samples of grantees/delegate agencies and children; participant recruitment procedures; random assignment procedures; definition and collection of outcome measures related to children, families, programs, and context; and data analysis plans regarding treatment impact, non-response bias, and community-wide effects. The report's four appendices describe the sample cluster stratification, describe ways to analyze program impacts without placing a 2-year exclusion on controls, discuss data sources for Head Start programs in "saturation" communities, and contain the impact research-related amendment to the Head Start Act 1998. (KB)