This paper reports on the impact of a professional learning programme on participating teachers' perceptions of mathematical reasoning. A total of 26 teachers participated in this study from four schools in Victoria, Australia and one school in British Columbia, Canada. The participants observed two demonstration lessons prepared and taught by the research team, attended pre- and post-demonstration lesson group discussions and taught each lesson in their classroom. Interviews with participating teachers before beginning the program, after trialling the first demonstration lesson, and after trialling the second lesson provided data for analysis. The Primary Teachers' Perceptions of Mathematical Reasoning Framework previously established by the research team was used to track the shifts in teachers' perceptions and understanding of mathematical reasoning across the program. We theorise that intentional foci on salient aspects of reasoning demonstration lessons, highly collaborative reflections, and teacher enactment of the demonstrated lessons have the potential to develop teachers' perceptions and understanding of reasoning.