This paper contributes to our understanding of teacher learning in the context of Lesson Study (LS), a model of professional development that involves collaborative lesson planning and evaluation. Video-recorded LS discussions of mathematics teachers based in London were analysed for this purpose. Two inter-related studies are presented: the first involved the construction of a reliable coding protocol for video analysis; the second used this protocol for coding 120 fragments of discussions amongst 91 teachers. Findings are discussed with reference to tests of reliability and results of multilevel analysis, which reveal differential effects of particular forms of interactions on learning processes.