In this study we investigated reading development from kindergarten to third grade for 1,913 economically disadvantaged children from the Early Childhood Longitudinal Study-Kindergarten Cohort. Characteristics of the child, the family, classroom instruction, and school composition were used to model influences from multiple levels of children's ecologies. The analytic model proposed that child and family characteristics (e.g., age at kindergarten entry, family literacy practices) would influence reading skills at kindergarten entry and rate of reading growth, whereas characteristics of classrooms and schools (e.g., comprehensive literacy instruction, racial and economic segregation) would constrain or enhance reading performance at specific times. Significant model parameters and effect sizes indicated that child and family characteristics were more predictive of initial reading skills than of reading development over time. Minority segregation in elementary schools was associated with lower student reading performance after accounting for child and family background, classroom instruction, and school-level poverty. (Contains 3 figures and 7 tables.)