There is perhaps no more pressing issue in school policy today than the achievement gap across social lines. Achievement differences between well-to-do children and poor children and between disadvantaged racial and ethnic minorities and majority whites are large when children first begin school, and they increase over time. Despite years of study and an abundance of good intentions, these patterned achievement differences persist, but who is responsible, and how are schools implicated? The increasing gap seems to suggest that schools are unable to equalize educational opportunity or, worse still, that they actively handicap disadvantaged children. But a seasonal perspective on learning yields a rather different impression. Comparing achievement gains separately over the school year and the summer months reveals that much of the achievement gap originates over the summer period, when children are not in school. The authors review Beginning School Study research on differential summer learning across social lines (that is, by family socioeconomic level) and its implications for later schooling outcomes, including high school curriculum placements, high school dropout, and college attendance. These studies document the extent to which these large summer learning differences impede the later educational progress of children of low socioeconomic status. Practical implications are discussed, including the need for early and sustained interventions to prevent the achievement gap from opening wide in the first place and for high-quality summer programming focused on preventing differential summer learning loss.