The goal of this causal-comparative, mixed methods study was to measure the degree of success that supplemental instruction (SI) promotes success and retention. The overarching goals of supplemental instruction are to improve student grades, reduce attrition, and increase student retention in a science, technology, engineering, and math (STEM) field. The study encompassed a review of traditional SI program structures, the theoretical underpinnings supporting SI programs, and the foundations of online supplemental instruction (OSI). Theories used to frame the study included constructivism, active learning theory, and Tinto's theory of student persistence. An OSI pilot was conducted in the Fall 2017 semester and discussed in terms of its implementation and the data collected. The quantitative and qualitative data collection procedures were discussed, and the analyzed results were compared to the existing literature. Finally, recommendations and implications to the field and future OSI implementations were discussed. [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.]