Early Head Start (EHS) is a two-generation program for pregnant women and families with infants or toddlers. Offered to those with limited incomes, its goal is to enhance children's development and health and to strengthen family and community partnerships. The Early Head Start Research and Evaluation Project (EHSREP), sponsored by the U.S. Department of Health and Human Services, Administration for Children and Families (ACF), was designed to answer questions about the overall impact of EHS programs and services on children and families and reveal how specific types of programs and services affect children and families that have different characteristics and life circumstances. Mathematica Policy Research led the rigorous evaluation, which was launched at about the same time the EHS program was authorized in 1995. In this data compendium, we provide a single source for information about the measures used throughout the EHSREP. We begin with an overview of the EHSREP design and then report the sample, data collection instruments, and response rates for each of three EHSREP data collection phases (birth to age 3, prekindergarten, and grade 5). Next, we describe the various data sets and documentation that are available to data users, and we provide a general description of how we have organized the more detailed information on the measures we used to create variables and scores for the public use data files. Appendix A contains detailed descriptions of the measures, including measure citations, publisher psychometrics and permissions, the wave or waves in which each measure was used, and information on the scales or variables in the EHSREP data set that were derived from these measures. (author abstract)