Science education research, reform documents and standards include scientific argumentation as a key learning goal for students. The role of the teacher is essential for implementing argumentation in part because their beliefs about argumentation can impact whether and how this science practice is integrated into their classroom. In this study, we surveyed 42 middle school science teachers and conducted follow-up interviews with 25 to investigate the factors that teachers believe impact their argumentation instruction. Teachers responded that their own learning goals had the greatest impact on their argumentation instruction while influences related to context, policy and assessment had the least impact. The minor influence of policy and assessment was in part because teachers saw a lack of alignment between these areas and the goals of argumentation. In addition, although teachers indicated that argumentation was an important learning goal, regardless of students' backgrounds and abilities, the teachers discussed argumentation in different ways. Consequently, it may be more important to help teachers understand what counts as argumentation, rather than provide a rationale for including argumentation in instruction. Finally, the act of trying out argumentation in their own classrooms, supported through resources such as curriculum, can increase teachers' confidence in teaching argumentation.