This dissertation is a mixed-methods study on the viability and impact of teacher-facilitated, school-embedded teacher professional development [TPD] programme to promote dialogue in primary mathematics in Chilean primary schools. Educational dialogue is conceived as a kind of talk that is both challenging and inclusive, giving students space to express and explore ideas collectively. Growing evidence supports its positive relationship with attainment, attitudes and reasoning. Still, internationally classroom talk patterns are usually non-dialogic, constraining students to a passive role. TPD is considered a key lever to bridge this gap. However, previous research often involves intensive, small-scale researcher-led interventions, which are costly and hardly scalable. This study aimed to (1) design and trial a programme to promote dialogic pedagogy in mathematics (2) understand its viability, and (3) assess its impact in teachers’ noticing, understanding and practices. The programme had built-in scalable features including semi-structured materials, low operational costs and local facilitators. Facilitators took part in an induction and then led 10-13 sessions with conceptual and practical components. Four schools initiated the implementation but only two of them finished (five participants in each). Data consists of pre-post lessons, pre-post video observations, interviews and teacher session recordings. Regarding viability, the programme was appreciated by participants, but two schools dropped out at different points in time. A tentative explanatory model indicated that these variations were linked to the leadership and research teams’ pressure and support, and facilitators’ ownership and commitment. In the meetings, facilitators’ actions were distinct from expert facilitation in the importance of management activities as well as their participation as peers. The peer-led TPD design tenets should, in consequence, be reconsidered to differentiate them from researcher-led models. Focusing on effectiveness, in the schools where the programme was implemented it led to positive results. Participants’ video observations shifted their focus towards dialogue. In the classroom and through interviews, participatory and elaborative aspects of dialogue were seen to increase, but not reasoning and challenging. Furthermore, teachers valued dialogue’s potential for improving inclusion and classroom climate. These results show that, where viable, the adopted TPD approach showed promise of being effective, while posing further questions about design and implementation conditions to favour scalability.