Concerns about the poor quality of students' use of sources in undergraduate research writing have typically led to investigations either of students' information-seeking strategies or of their composing practices. I argue that an either/or approach provides an incomplete picture of students' research writing processes, and that an exploration of the beliefs that shape students' use of sources is needed. This study explores the beliefs guiding undergraduate students in three disciplinary fields as they worked on a research writing assignment for a course in their majors. It seeks to understand what students, by their own accounts, believe a "good source" is, and how these beliefs shape the rhetorical decision-making in their own writing. Thirteen upper-level, undergraduate students enrolled at a private institution in the Southern region of the U.S. participated in this study. They completed two research questionnaires, took part in an in-depth interview about their strategies for using sources, and submitted a copy of their research papers. Analyses of the interview transcripts, the questionnaires and the use of sources in the research papers revealed that participants deferred to their sources in their writing, and that they relied on a turning-point source and conferred credibility to make decisions about sources. Participants assumed one of four positions in their use of sources: Organizer, Moderator, Framer, and Commentator. Most significant was that students approached research writing as a structured problem rather than as an ill-structured problem with no set solution. Thus, their search and writing processes were framed by dualistic/pre-reflective thinking (Perry, 1970; King & Kitchener, 1994). This study's results suggest that students' use of sources was grounded in their personal epistemologies of knowledge, which were, in turn, shaped by the instructional and situational contexts. I end with a call for instruction that focuses on how knowledge is constructed in students' majors-- expanding students' understanding of the rhetorical functions sources play in academic writing. I also call for further research on the role of personal epistemology in students' conceptualizations of sources, including how affect and instructional context shape students' use of sources. [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.]