Undergraduate writing fellows play an important role in administering writing assignments in writing-intensive courses. At the University of Michigan, the MWrite program was designed to support the implementation of writing-to-learn (WTL) assignments in STEM courses. Within MWrite, writing fellows are a primary instructional resource for students and help evaluate students' writing. As such, it is important to characterize writing fellows' beliefs about both WTL and writing more generally. In this study we interviewed writing fellows for MWrite courses in biology, chemistry, economics, and statistics about how they conceptualize WTL and writing quality. Our analysis indicates that writing fellows conceptualize WTL as supporting a range of content-focused learning outcomes and as featuring specific rhetorical elements that make WTL assignments successful. Most writing fellows discussed the importance of higher-order characteristics when evaluating the quality of students' writing, but also placed importance on the lower-order characteristics. Our results indicate that the writing fellows are internalizing the MWrite pedagogy with respect to WTL, but that their conceptions of writing quality appear to be informed by their experiences with writing more broadly. These findings support the use of writing fellows during the implementation of WTL in STEM courses that traditionally present barriers to using writing assignments. More generally, they indicate the potential for writing fellows' conceptions to support the aims of the writing fellows program of which they are part.