There is tension in the current curricular landscape, particularly for the field of English language arts (ELA). The majority of ELA teachers do not closely follow a district-adopted curriculum (Kaufman et al., 2020; Opfer et al., 2016). Further, recent studies by RAND found that nearly all teachers report supplementing with instructional materials they find on the Internet (Kaufman et al., 2018). Yet, researchers, thought leaders, and other influential organizations have asserted that adherence to a coherent and high-quality curricular program is an essential tool for equitable instruction and increasing student achievement. To untangle this tension, more research is needed to better understand supplementation. The purpose of this grounded theory study was to develop a theory about motivations that drive middle school English language arts (ELA) teachers' experience to supplement a district-adopted curriculum with other instructional materials. Intensive interviewing was used to explore the experience of supplementing among 12 middle school ELA teachers. This study offers a theoretical model--Self-Sustained Supplementation--as a framework to understand the motivations that drive middle school ELA teachers to supplement adopted curriculum. In essence, regardless of the quality of the adopted curriculum teachers have access to or the level of autonomy for curricular decision-making, they will continue to supplement because of the professional growth, joy, and collaboration that they experience from the process of supplementing. This study has significant implications for educators, teacher preparatory programs, and curriculum providers as they endeavor to support high-quality instructional materials for all students. [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://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]