This study explores questions of how educators learned about mathematics through lesson study but also how they were socialized into lesson study (LS) as a collaborative, routine practice. Specifically, the author compared the participation of educators who were new to lesson study (“LS novices”) with lesson study with those who had more experience with the practice (“LS experienced practitioners”). The author discovered a few key differences illustrate possible elements in the developmental progression of lesson study. Teachers who are newer to lesson study tend to focus on learning how to teach through problem solving, and seeing the collaborative work as a way to combine efforts to teach a better lesson. LS experienced practitioners, in contrast, were comfortable with the routine and can see their role as developing problems that elicit student thinking.