In this article, we report on the challenges of enacting a dialogic approach to teaching mathematics. Using the five principles and three repertoires of dialogic teaching as theoretical framework, we draw on interviews and classroom observations of a secondary-school teacher who aspired to teach mathematics through dialogue. We analysed his accounts and videos of his lessons to identify his strategies for dialogic teaching and challenges in implementing these. We found that the usefulness of strategies for dialogue changed over time and that some challenges were manifestations of tensions between the five principles, and thus intrinsic to dialogic teaching. Specifically, concerns for broad participation needed to be balanced against concerns for mathematical content, while the pursuit of a specific mathematical goal – the purposeful principle – lead to missed opportunities for chaining ideas into coherent lines of thinking and understanding – the cumulative principle
