In order to support more student-centered instruction in undergraduate science, as suggested by national reports over the last several decades, instructors may integrate undergraduate teaching and learning assistants (UTLAs) into their courses. A growing body of literature describes the beneficial outcomes of UTLA-faculty partnerships in teaching and learning, and opportunities for feedback, co-creation, and collaboration. However, scholars know little about what goes on during meetings between UTLAs and faculty to support feedback and collaboration, and have yet to investigate UTLA feedback in-depth. For this dissertation, I applied qualitative case study research methods to explore the nature of UTLA-faculty interactions and the quality and substance of the feedback provided to faculty by UTLAs. I studied the UTLAs and faculty instructors for two biology courses over the course of the Fall 2018 semester, collecting multiple sources of data, which included observational field notes, audio recordings of meetings, interviews, e-mails, and written documents. To explore the nature of UTLA-faculty interactions, I drew on the guiding principles of respect, reciprocity, and responsibility (Cook-Sather, Bovill, & Felten, 2014) to study how UTLAs were positioned in interactions with faculty. I found that UTLAs may be positioned as students, informants, consultants, co-instructors, and co-creators, that these positions were fluid and could occur simultaneously, and that respect, reciprocity, and responsibility manifested in various ways across these different positions. Thus, UTLA-faculty partnerships are complex and dynamic; even if we rank or characterize partnerships more broadly, considering the variety and fluidity in positioning may help to understand the nuances behind different types of partnerships. In addition to studying UTLA positioning, I also analyzed the quality and substance of the feedback the UTLAs provided to instructors, to explore if and how the feedback might play a role in formative assessment of student learning. I presented a conceptualization of UTLA-faculty interactions as part of a formative assessment “system” comprised of multiple feedback loops between instructors, UTLAs, and students. After analyzing the UTLA feedback, I found that UTLAs provided evidence about what’s going on with students in the course, and often, in addition to that evidence, provided interpretations, suggestions, and predictions to the instructor. UTLAs regularly offered feedback related to course logistics, and instructional materials. They also provided instructors with feedback on student attitudes, behaviors, and perceptions as well as student conceptual understanding. UTLA feedback was valuable for making adjustments to improve teaching and learning; however, UTLA feedback was not always related to or supported by evidence of student ideas. Thus, it was not always relevant for supporting deep formative assessment of student learning. Overall, this research helps to reveal new insights into the potential of UTLA-faculty partnerships for collaboration around instruction, formative assessment, and improving teaching and learning in undergraduate science, and how best to support those partnerships.