This paper focuses on how teachers guide construction of knowledge in classrooms. We suggest that guidance hinges on the kind of dialogue teachers choose to engage students in. We propose several classroom dialogue types relevant for the construction of knowledge and suggest that critical dialogue is particularly effective for knowledge construction. We describe a lesson on probability conducted in a Grade 8 classroom in order to illustrate how a teacher chooses dialogue types, and to what extent she attends during dialogue to epistemic actions, which are constitutive of knowledge construction. [For complete proceedings, see ED489597.]