Researchers, educators, and policymakers suggest the use of professional learning communities as one important approach to the improvement of teaching and learning. However, relatively little research examines the interplay of professional interactions (structural social capital) around instructional practices and key elements of professional learning communities (relational and cognitive social capital) from the lens of social capital. We employed a mixedmethod design that draws on social network analysis, surveys, interviews, and observations to understand what accounts for the pattern of professional interactions within four high-performing professional learning community schools. The results indicate that teacher trust, principal trust, professional dialogue with the principal, and years of teaching in the school play a significant role in explaining teachers' close network relationships. Implications and limitations are discussed.