We describe an urban school initiative aimed at teachers' professional development with the goal of increasing their mathematics content knowledge and helping them improve their practice. In the lowest performing schools, mathematics specialists were employed to teach only mathematics in upper-elementary grades (ages 9-12). One aspect of this initiative was a provision of time and space for the formation of site-based professional communities that were intended to support teachers in trying to implement changes in their practice. Teachers' professional communities developed at some sites and not at others. In this analysis, we explore the conditions that afforded or constrained the development of teachers' professional communities. Using two contrasting school sites as examples, we describe five aspects of the teachers' individual and collective professional lives that influenced the emergence of teachers' professional communities.