The purpose of this action research study was to evaluate the implementation of an adaptive web-based learning environment to personalize geometry instruction and practice for high school students at a private school in North Carolina. This study was guided by the following three research questions: How and to what extent does implementing personalized geometry instruction and practice through an adaptive web-based learning environment affect high school students' knowledge of geometry? How does using personalized geometry instruction and practice through an adaptive web-based learning environment affect students' motivation to learn geometry as well as their mathematics self-efficacy, anxiety, and self-concept? What are high school students' perceptions about using an adaptive web-based learning environment when learning geometry? This mixed methods action research involved the implementation of an adaptive web-based learning environment with 44 students in Grades 9-10 from three geometry classes. The implementation took place over a 7-week period, during the spring semester of 2021, and it covered two different geometry units: quadrilaterals and similar triangles. Geometry instruction was supported by integrating the adaptive web-based learning environment, IXL, into teaching as a supplemental piece as units were taught to students. It was used for individualized practice for students both in and out of the classroom. Quantitative data were obtained using a content knowledge pre- and post-assessment, as well as student surveys, and qualitative data were obtained using student interviews. Quantitative data were analyzed using descriptive statistics, paired sample t tests, and Wilcoxon signed-rank tests and qualitative data were analyzed using inductive/thematic analysis. Overall findings showed using the IXL platform as a geometry supplement helped students improve their learning of geometry. The majority of the students were found to have a positive opinion of IXL when used to help with learning geometry. Additionally, IXL helped improve students' motivation to learn geometry, mathematics self-efficacy, anxiety, and self-concept. [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.]