A growing body of research suggests that school leaders and policymakers should attend to the social conditions within schools that promote instructional improvement and student achievement gains. This dissertation uses theoretical and empirical work on social capital to frame three aspects of the relationships among teachers. The three studies offer distinct perspectives on the influences on and outcomes related to teachers' social capital. My first study examines how similarity in demographic characteristics is related to a teacher's perceptions of trust among his or her peers and how features of teachers' social networks influence that relationship. Through a series of regression models using a longitudinal data set, I find that demographic similarity only contributes to perceptions of trust in schools where teachers are more likely to seek advice from their peers of the same gender or race. However, these effects appear to be driven by selection such that teachers who perceive higher levels of trust work in schools with more peers of the same race. Using interview data from a sample of 34 first-time elementary school principals in Chicago, my second study examines how novice school principals conceptualize and strategize to build social capital among their teachers. I find that novice principals view their teachers' social capital as a means to improve their teachers' instruction via direct knowledge sharing and schools norms that support teacher development. I identify four strategies utilized by novice principals to develop their teachers' social capital and three constraints on the principals' ability to influence their teachers' instructional practice. In my third study, I argue that our understanding of professional community among teachers could be deepened by taking a more differentiated view of the construct. After detailing three limitations of the existing work on professional community, I use survey data from a school district in the midwestern United States to illustrate how these limitations can be addressed in empirical work. I examine how the communication patterns both within and outside of grade-level teams are related to the cooperative and learning dimensions of professional learning in elementary school grade-level teams. I further examine how one aspect of demographic composition, diversity in years of teaching experience, is related to the two dimensions. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]