<p>Mathematical argumentation in the classroom is demanding for students and their teachers. This article focuses on teacher actions and teachers’ shaping of mathematical argumentation. The results of our empirical study of Grade 8 classrooms show that, to understand how teachers’ actions shape mathematical argumentation, analysing mathematical argumentation processes as interactive processes is essential. Addressing “meta” knowledge is an attempt by teachers to provide a basis of argumentation, which turns out to be a complex negotiation process. The results of our study also indicate that other teacher actions can disrupt the mathematical argumentation process. The empirical insights revealed here complement previous work on teacher actions supporting mathematical argumentation in the classroom and indicate that shaping mathematical argumentation in class is complex and requires more than moderating argumentation. The role of formats of argumentation, which stems from an interactionist perspective, helps to address how teachers shape mathematical argumentation in class. We illustrate in this article how teachers support the development of a shared “basis of argumentation” and contribute to establishing such “formats of argumentation”.</p>