Background/Context: Seasonal researchers have developed a theory and hypotheses regarding the importance of neighborhood and school contexts for early childhood learning but have not possessed nationally representative data and precise contextual measures with which to examine their hypotheses. Purpose/Research Questions: This empirical study employs a seasonal perspective to assess the degree to which social context and race/ethnic composition--in neighborhoods and schools--affect the reading achievement growth of young children. The authors ask, Were there specific seasons when context and/or composition were particularly salient for reading achievement? Also, did accounting for context and composition challenge established appraisals of the relationship between family factors and achievement? Population: Data for a nationally representative sample of students proceeding through kindergarten and first grade came from the Early Childhood Longitudinal Study-Kindergarten Cohort (ECLS-K). Neighborhood social and race/ethnic measures came from the 2000 Census. Research Design: This quantitative study employs a three-level model that assesses reading achievement at school entry and during three subsequent seasons. The model represents reading achievement as a time-varying process at level 1, conditional upon family socio/demographic factors at level 2, and dependent on social context and race/ethnic composition at level 3. Findings/Results: Neighborhood social context mattered substantially for students' reading achievement levels at school entry and for their reading achievement growth during the summer. The proportion of neighborhood residents from minority race/ethnic groups was not associated with reading achievement at school entry or during the summer season. During the school year, school social context was associated with reading growth during kindergarten, and school social context and race/ethnic composition were associated with reading growth during first grade. Conclusions/Recommendations: The magnitude and frequency of contextual effects found in this national sample have considerable implications for achieving educational equality in the United States. The authors recommend that policy makers attend to the quality of neighborhood and school settings as a means of promoting literacy development for young children.