Peers influence children's social-emotional development and school engagement in important and unique ways. Recent research on peer social networks documents that children are affected by the nature of the school-based peer ecology, as well as by their personal peer experiences. Yet, little is known about how teachers affect the peer ecology, nor how teachers can promote positive peer influences in the school context. The four studies in this special section examine this issue. Together, they document the promise and the potential of studying the "invisible hand" of teacher influence on peer relations, and illustrate the need for further research in this area. This commentary considers the contributions of these papers for conceptualizing processes of transactional teacher and peer influence on student outcomes, and implications for interventions designed to reduce problem behaviors or increase school engagement. In addition, it considers the limitations of current knowledge and directions for future research. (Contains 1 figure.)