The lesson study model of professional development that originated in Japan is becoming increasingly popular in the United States. At its core, lesson study is a means of bringing teachers together to carry out the process of planning a lesson, implementing and observing it, and then examining it during a debriefing session. The debriefing component is one of the most distinctive characteristics of this type of professional development. It provides a means--discussion--for reflecting on the strengths and weaknesses of the collaboratively planned lesson. As such, the debriefing component merits special attention from those currently engaged in lesson study as well as those considering using it. This article describes debriefing session conversations that occurred during a lesson study project that established four lesson study groups (A, B, C, and D) in three high schools. The project brought university mathematics and mathematics education faculty together with high school teachers to collaborate on two lesson study cycles for each group. (Contains 2 figures.)