The purpose of this study was to better understand teacher tensions in professional development. The population under study was practicing mathematics teachers engaged in a week-long professional development institute. Data sources included observations, interviews, and teacher products (such as registration forms, surveys, journals, and notebooks). The data were analyzed to determine what tensions were evident in the institute and how those tensions arose. In this paper, we examine participants' tensions in a professional development situation when their expectations did not match reality. We have done so by presenting a theoretical model of professional development based on an instructional rhombus that illuminates the actual and applicatory aspects of teachers' professional development experiences. The main tensions expressed by the teachers were related to the content and pedagogy of the institute. This study points toward the importance of teachers' expectations in professional development.