This article overviews how professional development (PD) can help coaches support teachers in fostering classroom mathematical argumentation (CMA) for all students. After summarizing conceptual underpinnings of a PD program on coaching, the article describes the program, presenting a case study of how structures and tools designed by the PD for lesson planning and reflection informed a coach’s discourse with a teacher. Analyses of transcripts showed the extent to which a coach–teacher dyad discussed various topics related to mathematical concepts and practices and students’ participation during early and late coaching cycles, informed by the purposes and protocols designed to visualize, plan, and debrief lesson enactment. The article highlights recommendations for PD design and research to support mathematics coaches.