This mixed-method study addresses the need for a clear conceptualization of the professional reflection element of Lesson Study (LS), a popular collaborative approach to the professional development of teachers. Grounding and re-framing LS's post-lesson discussion in a theoretical framework of critical and collaborative reflection, we analyze the transcripts of four LS groups at German primary schools, focusing on depth of reflection and teachers' trajectories through their reflective practice. The findings show that LS groups differed significantly in the depth and the trajectories of their reflection processes. We consider implications for post-lesson discussions and critical reflection as a LS core skill.