This article discusses the place of thematic coherence in various approaches to educational dialogue and proposes a unifying approach to the analysis of thematic coherence of classroom conversations based on research in educational dialogue, philosophy of language and recent advances in linguistic research on discourse structure. Addressing the same main question is crucial to preserving thematic coherence in a conversation and is considered a key criterion of quality in classroom dialogues across the field of research in dialogic teaching, e.g., in Alexander's Dialogic Teaching, Michaels and colleagues' Accountable Talk, and Mercer's Interthinking. Nonetheless, a shared definition of thematic coherence is missing in the field. In order to provide such a conception, we propose a question centered approach to evaluating the thematic coherence of educational discourse. On this proposal, the thematic coherence of a conversation depends on how a conversational contribution relates to a conversation's overall question under discussion. We show how this approach may be used to examine thematic coherence and incoherence and argue that the proposal's focus on utterance level features of discourse has an advantage in helping researchers track how specific linguistic features of dialogue shape classroom conversations.