The purpose of this three study dissertation is to investigate why students are enrolled in introductory physics courses experience difficulties in being successful; one possible source of their difficulties is related to their epistemology. In order to investigate students' epistemologies about mathematics and physics, students were observed solving physics problems in groups during a laboratory course (study 1) and while solving physics and mathematics problems individually during office-hour sessions (study 2). The Epistemological Resources theoretical framework was employed (Hammer & Elby, 2002). Using emergent and a priori epistemological resource operationalizations (Jones, 2015), 25 distinct epistemological resources were identified in study 1. Differences in physics epistemological resource usage between students of varying academic background (as measured by their number of previously completed mathematics and science classes were identified. By employing an external (Jones, 2015) and internal (Scanlon, 2016) a priori epistemological resource coding scheme, a total of 17 distinct epistemological resources were identified in study 2. The data were sampled to compare the mathematics and physics epistemological resource usage of participants with consistent and inconsistent sign usage in an energy conservation physics problem in order to provide a meaningful context for discussion. Participants of the same sign usage group employed epistemological resources similarly. Conversely, participants in different groups had significantly different physics epistemological resource usage patterns. Finally, student epistemological resource usage patterns from the first two studies were compared to course outcomes in order to determine implications for practice (study 3). Educators must be aware of and address the epistemological underpinnings of students' difficulties in introductory physics courses. [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.]