This article describes the experiences of two US teacher groups and their lesson study work. Their stories illustrate some of the dynamics and promise of teacher change spurred by lesson study. The first case is primarily about teacher efficacy, as a group of teachers discover through lesson study that their planning and work can have an impact on student engagement in the classroom in a way that they had not realized. The second case is about one teacher's struggle with the move from isolation to collaboration, illustrating the tension between autonomy and collaboration that often occurs during such a transition.