Despite decades of research attention given to academic underachievement, longitudinal and developmental investigation of this phenomenon has been sparse. To address this shortcoming, the current study used a longitudinal, person-centered approach to identify latent subgroups of growth in the joint development of underachievement and four motivational beliefs (self-concept, task importance, psychological cost value, and self-worth) from first through sixth grade. Two types of underachievement latent classes emerged: one characterized by sustained underachievement and the other by growing underachievement (an Achievement class was also consistently found). Sustained, chronic underachievement was not associated with declines in self-concept or task importance, but was related to moderately lower levels of psychological cost value and self-worth, and was also related to lower middle school academic achievement. Growing underachievement was associated with lower and declining self-concept and task importance. Although differential class membership by gifted status was hypothesized, no such effects were found. Gender effects were found for the Task Importance and Self-Worth models in the hypothesized direction, but this effect was not as robust as in prior research. Findings from the current study build on prior research highlighting heterogeneity among underachieving students. v Dedication This dissertation is dedicated with love and deep gratitude to two very important people. …to my husband, Erik. Your patience, love, and support has sustained me throughout this journey, and I am forever grateful. None of this would be possible without you! …to Francia Barksdale. You touched the lives of so many gifted students over the course of your career, but most importantly, you inspired in me a profound passion for giftedness. I am so proud to follow in your footsteps.