Writing motivation has been deemed a critical aspect of the writing process because motivation affects how students engage with the writing process, how much effort and time they exert in the tasks, and the quality of their written products (Graham, 2018). Writing motivation has been the focus of recent studies on how to improve the discouraging levels of writing proficiency in young students, and the gaps between students from different racial, ethnic, and socioeconomic groups (National Center for Education Statistics, 2012). However, prior studies are limited in the methodologies used to characterize heterogeneity in writing motivation, the writing motivation constructs used to this end, the diversity of students represented in the samples, and the exploration of how writing motivation profiles relate to key motivational beliefs like achievement goals, and to writing performance. The present dissertation aims to address these gaps through a mixed methods exploration of what motivates a diverse sample of middle school students from a school district in the Mid-Atlantic region of the U.S. The study consisted of four phases. In Phase 1, I used latent profile analysis (LPA) to identify distinct profiles of writing motivation based on seven motives of writing: curiosity, involvement, emotional regulation, relief from boredom, grades, competition, and social recognition. The profiles were thoroughly described by comparing their demographic composition, scores in three achievement goal orientations (i.e., mastery, performance-approach, and performance-avoidance), and a concurrent measure of writing performance. Moreover, in Phase 2, I conducted semi-structured interviews with a randomly selected group of students from each profile and conducted a thematic analysis to complement students' quantitative evaluations of the seven motives with rich qualitative data. In Phase 3, I used a path SEM model to test the mediating role of achievement goal orientations in the relationship between the writing motivation profiles and a distal measure of writing performance. Finally, in Phase 4, I integrated the quantitative and qualitative results to generate meta-inferences about the profiles of writing motivation of diverse middle school students. Findings identified four distinct profiles of writers based on the seven writing motives. Students from all profiles valued all the motives to a certain extent, and grades and curiosity were consistently rated as top motives for writing. Nonetheless, integrated results suggested that three of the profiles were clearly driven by something specific. Personal Interest Driven Writers (PIDW) were motivated to write when the writing task was perceived as interesting or was perceived to bring personal benefits in the short- or long-term. Self-Expression Driven Writers (SEDW) were driven to write by a desire to express their emotions, ideas, and opinions and connect with the reader, and express overall positive attitudes toward writing. Performance Driven Writers (PDW) valued grades and competition as their strongest motives to write and wrote out of a desire to succeed over their peers and be recognized as top performers. On the contrary, Non-Specific Drive Writers (NSDW) were characterized by having high profile means in all seven motives, but no specific themes emerged to identify a specific drive for this seemingly high motivation to write. The profiles significantly differed in their demographic composition with the SEDW profile having more girls and Grade 6 students, the PDW profile having more boys and Grade 8 students, and the NSDW profile having higher percentages of students in special education and ELL status. Moreover, the profiles significantly differed in writing performance and achievement goal orientations. Furthermore, the SEM model supported the mediating role of performance-avoidance goals in the relationship between writing motivation profiles and writing performance. Notably, NSDW students had the highest performance-avoidance goals and strongest subsequent relationship to low distal writing performance. This study is the first one to employ a mixed methods design to systematically investigate what motivates diverse young adolescents to write in their ELA class, and the effects of heterogeneity in writing motivation on their performance in an argumentative writing task. The findings about the distinct profiles based on motives, and the significant differences in demographic composition, achievement goals, and writing performance, have significant implications for researchers of writing motivation and for practitioners who aim to motivate students as a means to enhance their writing performance. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]