Middle childhood is an important but often overlooked developmental period. Similarly, a large literature examines the importance of child care as a developmental context, focusing almost exclusively on pre-school child care experiences. Through three related empirical studies, this dissertation aims to address these two gaps in the extant literature. These complementary studies and innovative methodologies used make a substantial contribution to the middle childhood and child care literatures. In addition, this research has important implications for policy, practice, and future lines if inquiry. Further, this dissertation introduces developmental researchers to an important dataset, the Early Childhood Longitudinal Study Kindergarten (ECLSK) Cohort, and describes the benefits of using this dataset to address developmental questions. This dissertation examines the development of and interplay between achievement, attention, and behavioral trajectories over the first six years of schools and the role of school-age child care in child well-being. The first study, using growth modeling, examines the relationship between academic, attention and behavioral trajectories, finding that increases in attention skills across the kindergarten year are related to increased gains in achievement in kindergarten, first, and third grades. This study also finds no evidence of a relationship between behavior and achievement trajectories. The second and third investigate the relationship between school-age child care experiences and child well-being. One study uses longitudinal regression and propensity score methods to estimate the effect of center child care during the kindergarten year on child academic and socioemotional skills. Findings from this study indicate that a positive relationship between center care and child externalizing behaviors is both consistent and robust across a range of models and methods. The second study examines the relationship between type and extent of child care during both the summer before first grade and the first grade year and child well-being. This study uses longitudinal regression and multiple imputation techniques. Findings from this study indicate that the type of child care and not the extent of care relate to socioemotional skills. Center care during the summer prior to first grade is associated with increases in externalizing behaviors. (author abstract)