In this research report we share results of a study of an adaptation of Lesson Study, a form of professional development typically used in Japanese elementary schools, with secondary mathematics teachers in the United States. We draw on Wenger's (1998) concept of "communities of practice" and Grossman, Wineberg, and Woolworth's (2001) discussion of "pseudocommunity" to present our analysis of two types of activities: a cycle of lesson development, implementation, and revision as it occurred in one lesson study group (LSG), and book discussions that occurred in large group meetings all LSGs. Our analysis suggests that activities such as these can support community of practice development and that "community development" is useful as a concept for structuring and studying mathematics teacher professional development. [For complete proceedings, see ED500858.]