Genre has emerged as a central concept in writing studies, with numerous scholars advocating for its prominent role in writing instruction. Despite this interest in genre, however, research has not explored teachers' understanding of the concept, which is critical to how they address genre in their classrooms. This study traces the evolving conceptions of genre among thirty-three new first-year writing teachers, examining their understandings--and, occasionally, tensions--at different points in time as they encounter the concept in their teacher preparation and with their own students. Through written reflections and focus group interviews, we identify key patterns in how the teachers define genre over time and some of the influences on those dynamic conceptions. Findings from this research have implications for teacher preparation and curriculum development in the context of U.S. college composition.