Background/Context: Researchers have proposed a number of lenses for analyzing teacher professional communities in recent years. These lenses have been useful in describing key dynamics of professional communities; however, none provides a compelling approach to how to integrate data from the school as a whole with case study data on individual interactions to create a coherent account of the structure and dynamics of teacher professional communities. Objective: Our objective was to present and illustrate the application of social capital theory for analyzing the role of formal and informal teacher interactions in helping teachers enact changes to instruction associated with ambitious school reforms. Social capital theory posits that valued resources and expertise are embedded within social networks and that it is through social ties that one gains access to and can make use of resources to effect change. The network perspective directs researchers to focus simultaneously on the overall social structure of a school and on the expertise and resources exchanged through interactions among teachers that take place in meetings, staff rooms, hallways, and classrooms. Setting: Our illustrations are contrasting cases of teacher communities in two elementary schools in California. In both school communities, the principals were committed to the idea of fostering greater interaction among colleagues as a strategy to improve literacy instruction. Both schools had similar levels of resources to support their goals through external funding from the state, but the schools had had different levels of success in implementing their reforms at the time of the study. Research Design: We used an explanatory case study methodology that relied on social network, survey, and interview methods as sources of evidence for several alternative hypotheses relating to how the distribution of resources and expertise may have contributed to these schools' different levels of success in implementing their reforms. Conclusions/Recommendations: The comparative case analysis of the two schools provided evidence that analyzing the internal structure of the school community was necessary to help account for the distribution of access to resources and expertise in these two schools. Moreover, there was some evidence from survey and network data that the distribution of valued resources and expertise was related to the level of change observed in each school.