Among the best examples of the professional development of teachers that has become a key component of systemic educational reform are the curriculum-based teacher networks that have been created and nurtured in public schools by external private and public funders. This study examined the impact of four such networks on teacher renewal and systemic school reform in the school district of Philadelphia (Pennsylvania), the nation's fifth largest school district. The networks chosen were the largest, longest-lived, and most well-developed of Philadelphia's networks: (1) the Philadelphia Writing Project; (2) the Urban Mathematics Collaborative; (3) Science Resource Leaders; and (4) the World History Project (and its spinoff, Women in World History). Information came from document review, observation, interviews with 5 teachers from each network, and a survey completed by 193 teachers over all networks. Focus groups further explored the development and operation of these networks. Findings support the conclusions of other investigators that teacher-centered subject-area networks provide powerful learning experiences for their participants. Similarities and differences in the networks are traced. Seven tables (including graphs) present study findings. (Contains 33 references.) (SLD)