This article presents research focused on establishing ways that dialogic talk between teachers and students promotes mathematical reasoning in early years classrooms. Data are drawn from recorded and transcribed Year 1 mathematics lessons. Conversation analysis provides close examination of the talk-in-interaction practices of teachers and students and reveals how mathematical reasoning is co-produced through the turn-by-turn exchange structures between teachers and students. Analysis of selected transcripts illustrates how different teacher talk moves create a dialogic space that make student mathematical reasoning possible. Implications for classroom practices are made.