Children with higher levels of executive function (EF) skills consistently demonstrate higher levels of academic achievement. Despite the consistency of these associations, fundamental questions remain about whether efforts to improve an individual child's EF skills result in corresponding improvements in his or her academic performance. In the absence of experimental evidence, developmentalists have used repeated measures designs to test the nature, magnitude, and direction of the associations between EF skills and academic achievement. In contrast to previous studies, this study described how between- and within-person associations between EF and achievement address different questions. Using data from a subsample of participants (N = 6,040) from the Early Childhood Longitudinal Study-Kindergarten, 2010-2011 (ECLS-K:2011) cohort, we estimated a series of latent growth curve models with structured residuals to test the between and within-person associations between 2 dimensions of EF (working memory, cognitive flexibility) and 2 domains of academic achievement (math, reading). Whereas between-person associations between EF and achievement were large (φ = .55-.91), the within-person associations were small (βs = -.10-.25). Within-person effects of earlier reading achievement on later EF skills was the most consistent finding. Results were unchanged when analyses were repeated using the subset of children who were eligible for free and reduced-price lunch, a proxy for low socioeconomic households. Results are discussed with respect to interest in improving EF skills as a means for facilitating school outcomes. (PsycINFO Database Record (c) 2019 APA, all rights reserved).