Mathematics teachers participating in Lesson Study: Two case study sites' is research which tracked the introduction of school-based Lesson Study model of professional development to two post-primary schools in Ireland. This research took place in the academic year 2012-13 and follows the participation of two groups of teachers in two case study schools: Doone and Crannog (pseudonyms). All names have been anonymised. This data refers to three interviews with Dave in Crannog school undertaken across the academic year.