Large class sizes can be a barrier to seeking assistance, inhibiting student success. Efforts to improve student outcomes and address the reluctance of students to access academic resources lead many institutional leaders to turn to unique academic support initiatives, like supplemental instruction (SI). There is ample evidence that supplemental instruction improves student success. However, there is limited research on the effects of SI in high-enrollment courses. Framed through the lens of social constructivist theory, the purpose of this qualitative exploratory case study was to: (1) examine the impact of supplemental instruction on the course performance of students enrolled in several high-enrollment introductory social science courses; (2) discover how SI attendance impacted course outcomes, and; (3) explore what components of SI participants perceive as beneficial and how they perceive those benefits impacted their course outcomes. An analysis of archival academic data showed that students who participated in SI had higher mean course grades, lower probation rates, and lower course failure rates. Moreover, this study provided evidence that the frequency of attendance also helps to improve course performance. A comparison of students who attended the most versus those who attended the least showed the greatest improvement. Additionally, data from semi-structured interviews revealed that SI could help students navigate high-enrollment courses by fostering a smaller learning environment. Students felt SI sessions allowed for more personal interactions with SI leaders and their peers. Moreover, students indicated they felt an increased sense of confidence and empowerment. Key recommendations for practice include piloting SI programs for traditionally difficult courses with high drop-fail-withdraw (DFW) rates, designing robust programs that emphasize collaboration amongst program staff, faculty, and students, and using SI as an academic support resource in high-enrollment courses. Recommendations for future research include designing a more robust qualitative study with a larger interview sample and designing a multivariate analysis to control for student characteristics that might affect course grades and lead to differences in the likelihood of taking advantage of the SI program. [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/en-US/products/dissertations/individuals.shtml.]