This study investigated the nature of interaction among teachers that occurred around the National Board certification (NBC) process and how that collegial interaction influenced teachers' professional development. Qualitative interviews were conducted with 14 teachers who were either considering NBC, in the process, or had been awarded NBC. Interview data were analyzed using the constant comparative method. Results indicated that teacher interactions triggered by NBC helped one another's professional development in several ways: (a) enhancing reflection on teaching practice, (b) establishing a professional discourse community, (c) raising the standards for teaching performances, and (d) facilitating collaboration. Implications for teacher professional development were discussed.