Collective efficacy, a group's belief in its capabilities to reach a goal, is an important organizational property repeatedly linked with student achievement. However, little scholarship specifies the antecedents of collective efficacy. To fill this gap, this study examines a potential predictor of collective efficacy: teachers' social networks. The authors employ social network and regression analysis to explore the relationship between network density, network centralization, and collective efficacy in 20 middle school mathematics departments in two large, urban districts across 3 years. Collective efficacy had a significant relationship with density, but not centralization, when controlling for school demographics. The findings underscore the importance of network density to school improvement reforms. Policymakers need to consider policies that support the building of a dense network, which could increase collective efficacy and, ultimately, student achievement.