We here add to the sparse literature on the use of Lesson Study (LS) in initial teacher education (ITE), reporting how LS can mediate development of reflective practice (RP). A cohort of 50 student-teachers, all secondary mathematics postgraduates, were involved in lesson study subgroups: planning, teaching-and-observing, analysing and reflecting on observations, and re-teaching. Each study group worked on their own LS in selected schools with teachers/school based mentors that had some previous experience of lesson study. We tutors/researchers/authors observed the planning, teaching and post-lesson analyses, and one of us interviewed selected participants. Two main findings were: (i) the significance of imagination for reflective practice, here prompted by the focus on improving and ‘re-teaching’ the lesson; and (ii) the importance of the ITE-peer group and its relations with more powerful others (mentors and tutors) to development. We conclude that LS may complement and even ‘lead’ the development of reflective practices of student teachers, providing a Zone of Professional Development.