Geometry education is an important aspect of STEM education and career development, but it is often overlooked in K-12 education in the United States. Although there is some research on teaching geometry to students with learning difficulties at the elementary level, there is a lack of research on teaching advanced geometry skills at high school levels. Chunking is a strategy that can help reduce the cognitive load demanded in the processing of information, and it has been applied as a testing accommodation for high school students with math learning difficulties in geometry assessments. This study aims to expand upon the existing literature by examining the effect of chunking as an intervention, rather than just a testing accommodation, on the geometry performance of high school students with math learning difficulties. The study used a multiple probe design across participants and address the following research questions: (a) To what extent will the chunking intervention improve the geometry performance of high school students with math learning difficulties as measured by a proximal measure (i.e., geometry problem solving probes related to lines and angles)? (b) To what extent will the effects of chunking intervention maintain two weeks after the conclusion of the intervention as measured by the proximal measure? (c) To what extent will the students be able to generalize their geometry knowledge to distal measures, such as geometry proof related to lines and angles, sample items from the NJSLA high school test preparation for geometry, and the KeyMath-3 geometry subtest? (d) What are the perspectives of the high school students with mathematics learning difficulties about the chunking intervention? Three high school students with difficulties in learning geometry participated in this study during an in-school intervention program at a high school in New Jersey. The intervention was implemented by the interventionist and a trained undergraduate research assistant, and consisted of 6 sessions, divided into three units: (a) a unit on angles and related properties, (b) a unit on parallel lines (c) and a unit about perpendicular lines. The entire intervention, including the baseline and post-test phases, lasted approximately two months, depending on the participants' availability and their sequences of introduction into the intervention. The treatment's fidelity was monitored and reported, and students' perspectives on the chunking intervention was evaluated through a social validity measure at the conclusion of the intervention. The outcomes of visual analysis indicated that participants improved their performance during the intervention phase as compared to the baseline phase in the geometry problem-solving test's proximal measure. Compared to their corresponding scores in the baseline, most of the participants' scores in the posttest showed significant enhancements on the transfer measures. Participants who underwent the maintenance tests sustained elevated performance levels even two weeks after the conclusion of the intervention. Individual differences in students' responding to the intervention were discussed with regard to their pre-requisite skills and conceptual knowledge of the geometry concepts. The researcher then discussed the limitations, contributions to the literature, implications for practice, and future research in teaching geometry, and recommended that chunking strategy should be integrated with, rather than replacing, concept instruction. [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.]