This article reports survey and interview research on how graduate student instructors (GSIs) across the United States navigate the boundaries of disciplinary expertise that define their work as students and teachers. The disciplinary backgrounds of GSIs in this study influenced their experiences with formal writing pedagogy education and their teaching practices. GSIs imported content, mindsets, pedagogies, and skills and expertise from their home disciplines into the FYW classroom and exported practices and dispositions from FYW into their own work as graduate students. I suggest how writing pedagogy educators might reframe preparation experiences to recognize the disciplinary boundaries GSIs work across and to repurpose these boundaries as sites for richer professional development and writing instruction.