The term summer setback refers to an erosion of children's numeracy and literacy skills over the summer months. While children tend to gain skills at equal rates during the school year, differences in educational achievement between advantaged and disadvantaged children grow during summer vacations. Leading researchers estimate that by ninth grade, achievement gaps between advantaged and disadvantaged children are mainly rooted in summer learning differences and that these gaps predict high school tracks, graduation and noncompletion rates, and postsecondary attendance. Summer setback research has made a substantial contribution to our understanding of the family and school sources of educational inequality and has encouraged the development of program interventions.