In this chapter, I examine argumentation from the perspective of dialogic argumentation that highlights students’ engagement with each other’s ideas in the process of making mathematical claims and providing evidence to support them. I analyzed the provided data for students’ dialogic and justifying moves and investigated how the teacher supported argumentation. I found that the students’ dialogic moves included elaborating and commenting on peers’ ideas. Students’ justifying moves included one case of articulating reasoning and several instances of describing support. The teacher structured argumentation by sequencing it into steps so that each step established a new piece of information. This may have restricted students’ need to articulate reasoning and to challenge or ask questions about the presented ideas. The teacher’s dialogic communicative approach seemed to support students in elaborating each other’s ideas. On the other hand, use of an authoritative approach appeared to constrain students’ possibilities to provide support for claims. Considering dialogic aspects enriches the analysis or argumentation when argumentation is understood as a process as emphasized in the definition of argumentation used in this book.