Two departments in a secondary school in England participated in "lesson study" projects over a five-month period to explore its usefulness as a vehicle for professional development. Through a cycle of two research lessons, conducted separately in each department, teachers identified challenges that inhibited the learning of their students and collaboratively prepared innovative approaches to address the learning challenges. The process yielded multiple sets of data: DVD-recorded lessons, lesson plans and resources, transcripts of preparation and evaluation meetings and individual interviews at the end of the project. This paper draws principally on detailed qualitative analysis of end-of-project interviews about teacher experiences of learning and practice development in the two contexts. While the teachers encountered some logistical challenges to the implementation of lesson study, a number of important gains were reported: collaboration in lesson study reduced feelings of professional isolation; teachers reported a sharper focus on pupil learning and more confidence to take risks with approaches to teaching, which led to greater opportunities for pupils to engage in interactive activities, for example, involving problem-solving and peer teaching in groups.