Lesson study is highly regarded as a model for professional learning, yet remains under-theorised. This article examines the professional learning experiences of teachers and numeracy coaches from three schools in a local network of schools, participating in a lesson study project over two research cycles in 2012. It maps the interconnections between their experiences and their beliefs and practices, using Clarke and Hollingsworth's (2002) Interconnected Model of Professional Growth. Analysis of interview data and video-recordings of planning meetings, research lessons, and post-lesson discussions reveals the development of teachers' collaborative planning skills, increased attention to students' mathematical thinking, use of orchestrated whole-class discussion based on anticipated student solutions and focused questioning, and the enhancement of collaborative practices for teacher inquiry. Our findings illuminate the interplay between the External Domain, the Personal Domain, the Domain of Practice, and the Domain of Consequence, in the teaching and learning change environment, and the mediating processes of enactment and reflection. Changes in the domains across the period of the lesson study provide evidence of teachers' professional growth, with the iterative processes of enactment and reflection being critical in mediating this professional growth. Author abstract, ed