This article presents research on how teacher developers in the United States learn to conduct lesson study. Although the practice of lesson study is expanding rapidly in the US, high-quality implementation requires skilled facilitation. In contexts such as the United States where this form of professional development is relatively novel, few teachers have participated in lesson study, so leaders of lesson study groups do not have that prior experience to draw upon for facilitation. To establish lesson study groups, teacher developers are therefore needed in the US context, but we know little about how leaders who are new to lesson study learn to do this work. To investigate this, two novice teacher developers were followed for a period of eighteen months, from their first exposure to the literature on lesson study, through their participation in lesson study conferences, apprenticeship with an experienced lesson study leader, and into their independent conduct of lesson study groups. Data show that the facilitators learned to contend with such issues as teacher resistance, the use of time, and the shifting imperatives of directing teachers' work versus stepping back to give teachers autonomy in determining their collective work. The article concludes by suggesting that lesson study functions as a countercultural bulwark in the field of teacher learning by promoting a participant-driven, time-intensive form of professional development, and that, despite its novelty and complexity, teacher developers with strong mathematical and pedagogical backgrounds become reasonably skillful facilitators in a surprisingly short span of time.