Lesson study is a form of teacher professional development that is intrinsic to the Japanese educational system in first and second level schools and in teacher education. Lesson study has been credited with the success of Japanese pupils in international comparative tests of mathematics achievement (Stigler and Hiebert, 1999). It is gaining international credibility as a means of enhancing the scholarship of teaching and promoting mathematical achievement in diverse school cultures (Asia-Pacific Economic Cooperation Education Network, 2008). Lesson study is a deceptively simple protocol with highly textured nuances. Each lesson study cycle involves a group of teachers, working collaboratively, and hinges on the detailed preparation of a research lesson, which is taught by one of the group and observed and reviewed by others. Increasingly, lesson study is being recognised as an inherently complex site of social, situated and distributed learning (Lave and Wenger, 1991) which challenges the researcher to find new markers of how and under what conditions, participation in the practice of lesson study builds mathematics teacher capacity and translates into more successful teaching of mathematics. Two conjectures have been formulated explaining why lesson study improves teaching and inviting research into the process (Lewis, Perry and Murata, 2006). The study on which this presentation draws consisted of three tiers (Corcoran, 2008). In the third tier, a teacher development experiment was designed and implemented using lesson study on a yearlong education elective course to develop mathematics for teaching. In this presentation, the concept of "communities of practice" (Wenger, 1998) is used as a heuristic to examine notions of "engagement," "alignment" and "imagination" in relation to learning about teaching mathematics on the part of the six student teacher participants. "Accountability to the enterprise" of lesson study and the development of a "shared repertoire" facilitated the "negotiation of meaning" of "research lessons." (Contains 1 figure.) [For the full proceedings, "Research-Teaching Linkages: Practice and Policy. Proceedings of the Third Annual Conference of the National Academy for the Integration of Research, Teaching and Learning (3rd, Dublin, Ireland, November 11-12, 2009)," see ED539248.]