Accurately measuring and modeling academic achievement growth is critical to support educational policy and practice. Using a nationally representative longitudinal data set, this study compared various models of mathematics achievement growth on the basis of both practical utility and optimal statistical fit and explored relationships within and between early and later mathematics growth parameters. Common patterns included a summer lag in achievement between kindergarten and Grade 1 and an association between achievement at kindergarten entry and later achievement. Notably, there were no statistically significant relationships between early and later rates of growth, and there was minimal variability in achievement growth in the late elementary and middle school grades. Challenges related to assessing academic achievement in the middle grades and modeling academic skill development are discussed.