Declining mathematics scores are forcing school administrators to look for nontraditional ways to educate students. Adaptive learning systems have been implemented in schools with the purpose of providing personalized learning opportunities for students. The purpose of this experimental study was to investigate the effect of adaptive supplemental lessons on elementary students' achievement in mathematics. This study was conducted in a large, urban school district. This study utilized a quasi-experimental research design with a sample size of 7,114 students in Grades 3-6 in the control group (2015-2016 school year) and 7,733 students in Grades 3-6 in the treatment group (2016-2017 school year). The treatment group received supplemental adaptive mathematics lessons within their regular mathematics block. The adaptive learning system used was LGL Math Edge. This study also included analysis of the effects of adaptive learning on various subgroups, as well as the relationship between time spent in LGL Math Edge and mathematics achievement from pretest to posttest. Secondary data were used to compare the mean gain score of the adaptive, diagnostic mathematics assessment (ADAM) pretest and posttest between the control and treatment groups. The results of this study showed that there was a statistically significant difference in student achievement when students received the treatment of adaptive learning. This was true for all subgroups examined. The results of this study also showed there was a statistically significant relationship between the amount of time students spend in LGL Math Edge and mathematics achievement on ADAM. The findings thus revealed that using supplemental adaptive lessons in addition to conventional instruction improved student achievement in mathematics. [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.]