The goals of the Head Start Impact Study (HSIS) are to: (1) determine the impacts of Head Start on children's school readiness and parental practices that support children's development; and (2) to determine under what circumstances Head Start achieves its greatest impacts and for which children (US Department of Health and Human Services, 2010). The HSIS includes a nationally-representative sample of newly entering 3- and 4-year old children from 84 grantee/delegate agencies who were randomly assigned to either have access to Head Start or to a control group who did not have access to Head Start but could enroll in other early childhood programs. Results of prior analyses that address the first goal of the HSIS found benefits for children at the end of Head Start that were concentrated on language and literacy skills for the 4-year-old group and benefits at the end of Head Start on cognitive, social-emotional, health, and parenting outcomes for the 3-year old group. However, the early advantages found at the end of Head Start on most of these outcomes were non-significant by the end of kindergarten and first grade. The second goal of the HSIS is to identify the circumstances under which Head Start achieves its greatest impacts, and one possible explanation for the largely non-significant effects at the end of kindergarten and first grade has to do with the quality of children's experiences in elementary school. The purpose of this presentation is to describe the methods used to examine the extent to which the impacts of Head Start on children's cognitive and social-emotional outcomes at kindergarten, first grade, and third grade may be moderated by the quality of children's experiences during elementary school.