A widespread instructional practice in the teaching of argumentative writing is the use of writing samples or models during instructional conversations about what counts as “good argumentative writing.” In this article, we focus on a set of lessons in a high school English language arts classroom in order to gain insight into how a teacher’s use of writing samples contributed not only to what counts as “good argumentative writing” in that classroom, but how her instructional conversations with a group of 11th grade students revealed a shift in her “argumentative epistemologies” for teaching literature-related argumentative writing. We examine simultaneously the impact of instructional conversations on the evolution of the socially constructed definitions of good argumentative writing and the teacher’s shifting argumentative epistemologies by tracing the teacher’s socially constructed definitions of good argumentative writing in the instructional conversations around argumentative writing samples as they evolve and change over time. The findings suggest that the interactional construction of “good argumentative writing” in instructional conversations was influenced by the broader instructional context (e.g., a testing regimen), by the teacher’s argumentative epistemologies for what counts as good writing, and by the conversational interaction of teachers and students in classroom lessons.