Science, technology, engineering, and math (STEM) undergraduate research programs have been shown to impact students' access, learning, and retention in STEM fields (Reave, 2004; Seymour, Hunter, Laursen, & DeAntoni, 2004). Research has also shown that student-adviser relationships in STEM undergraduate research programs prepare students for graduate education and their future professional careers through professional, intellectual, and social support. However, what is not yet clear is the role of writing in STEM undergraduate research programs. While scholars in writing studies have explored the teaching and learning of scientific writing in undergraduate education, little is known about the role that writing in undergraduate research programs play in students' disciplinary identity formation. Rather, studies have focused on how undergraduate students learn to write disciplinary or professional genres (Dias, Freedman, Medway, & Pare, 1999; Poe, Lerner, & Craig, 2010; Winsor, 1996). This study explores how students participating in an undergraduate research program used writing in their daily research and what role writing played in their disciplinary identity formation. In this study, I followed four students participating in a 10-week summer undergraduate research program at a STEM-focused research university. I traced students' writing activity and perception of writing as they wrote their research proposals, which was part of the application process to be accepted as student researchers in an existing laboratory or research group. Next, I traced students' writing activity, social interactions, and how they perceived and used writing during their 10 weeks working in their respective lab or research group. The four students came from different STEM fields and class standings: a freshman considering a major in computational chemistry, a senior bioengineering major, a sophomore math major, and a senior chemistry major. I conducted discourse-based interviews (Odell & Goswami, 1985) with these students, collected documents of their writing throughout their participation in their respective research group or lab, and conducted a screencast video analysis (Takayoshi, 2016) of each of them writing a required research progress report. I also used ethnographic interviewing methods to understand the social nature of each student's research lab or group. My findings show that students who participated in a summer undergraduate research program where they had more opportunities to interact with other members of the lab, observe the dynamic activity and collaboration of STEM research (e.g., attend lab meetings, work alongside other members of the lab), and participate in dialogic conversations with other members of their lab to discuss disciplinary research articles showed a greater sense of belonging in their research group than those who did not participate in a summer undergraduate research program. These students also wrote more small genres in addition to the assigned STEM genres such as the progress report and final technical report. These self-initiated small genres played a role in students developing their scientific rhetorical awareness as they communicated their data to other members of their research group. These students also began to use writing as part of their daily research activity--to document and analyze the work they conducted in their research lab or group, or as a means to make sense of the literature in their field. Based on these findings, I developed a model for communicative writing contexts that provides STEM educators a lens to develop pedagogically effective support systems for STEM undergraduate research programs. My findings suggest that integrating writing into existing STEM research activities or developing new opportunities for writing can contribute to students' disciplinary identity formation. [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.]