The purpose of this action research was to investigate how the use of technology promotes interaction to foster high school students' mathematical understanding. This mixed method study is guided by social-constructivist theory (Vygotsky, 1978) and framed within Moore's (1989) model of learner-content, learner-instructor, and learner-learner interaction. The participants in this study included 11 high school mathematics students enrolled in a Web-enhanced Advanced Placement Calculus course. Analysis of coded observational data, field notes, and interviews with participants provided insights about technology facilitated interaction and mathematical understanding. Participants completed a 36 item course evaluation and an 89 item interaction questionnaire. The emerging themes were supported with quantitative data derived from participant responses. The qualitative and quantitative results of this study revealed that: a) Online threaded discussions served as an extension of the face-to-face classroom; b) learner interaction with content, the instructor, and other learners was sustained at high levels throughout the course; c) online interactions enhanced interaction in face-to-face settings; d) students perceived the AP Calculus course as an active learning environment; e) online activities impelled learners to interact, more than they would in a traditional course, with the instructor and other learners; f) technology facilitated collaborative activities encouraged learners to interact with content at deeper levels; g) structured learner-learner interaction was a transformational experience for some learners; h) the online course and included collaborative activities supported learners' mathematical understanding. [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.]