Purpose: Teacher leadership distinguishes, implicitly or explicitly, among teachers based on their expertise, but the notion of teaching expertise is contested, even among educators. Despite the potential for expert teachers to positively influence their colleagues' practices, we know little about the supports and obstacles to expert teacher leadership. This study examines the ways that the leadership of a particular group of expert teachers--National-Board-certified teachers--is understood and enacted in schools. Methods: Survey data from staff in six elementary schools were used to analyze Board-certified teachers' centrality in their schools' work-related social networks. Survey results were used to select 26 participants for semistructured interviews focused on their understandings of the relationships between teaching expertise, Board certification, and leadership, which were analyzed using alternating rounds of open and closed coding. Findings: Board-certified teachers were more central to their schools' networks than non-Board-certified teachers, although Board-certified teachers interacted with one another more than with non-Board-certified teachers. Board-certified teachers were seen as having expanded influence beyond their classrooms, which some saw as supporting their leadership, but others saw as disconnected from, or even undermining, their leadership. Implications for Research and Practice: Teacher leadership requires a supporting infrastructure in the form of leadership positions, routines, and tools that align with leaders' expertise. Developing a shared understanding of teaching expertise is key to Board-certified teacher leadership, as well as other teacher leadership efforts.