This study addresses the mathematics that student teachers bring to primary teacher education and examines how their mathematics might be developed. The study is conducted in three tiers. Tier One examines the mathematical knowledge for teaching of a cohort of second year B.Ed students using a self-audit designed for the purpose in the UK. Tier Two studies the actual teaching of mathematics of a sample of second year student teachers during their spring teaching placement. Tier Three describes a mathematics teacher development intervention in which six students chose to participate during their final year B.Ed programme. The intervention consisted of forming a Community of Practice dedicated to learning how to teach primary mathematics well. The Community of Practice adopted Japanese Lesson Study as a model for teacher development and three cycles of lesson study were pursued over the course of the study. In each cycle, student teacher participants prepared mathematics lessons collaboratively. Then, two students each taught a research lesson in different schools, observed by other students who studied children’s mathematical response to the research lesson. This experience was followed by a reflective meeting to analyse the lessons using the Knowledge Quartet framework as a means of focusing on different dimensions of the mathematical content of the lesson. I was party to the lesson study process as researcher and as Knowledgeable Other. Community members also interrogated their previous, possibly negative relationships with mathematics and engaged in doing mathematics as part of the lesson study enterprise. Indications are that participants in a community of practice where the members engage in lesson study results in enhanced mathematical content knowledge for teaching and the development of a mathematics teacher identity. To engage in Lesson Study is to learn mathematics for teaching.