Background: Communities of practice (CoP) have been proposed as a promising form of continuing professional development for teachers' learning. However, there remains limited understanding of the processes and factors that sustain teacher-initiated CoP. Purpose: This study provides a qualitative examination of a group of internal leaders' perspectives on how a large teacher-initiated CoP in South Korea was established and sustained as an effective form of continuing professional development. Methods: Data included semistructured interviews with the internal leaders, Website artifacts, and researchers' reflective journals, and were analyzed through inductive analysis. Results: Findings included the following: (a) shared passion for good teaching, (b) widening professional network, (c) collaboration through sharing, and (d) continuing professional development opportunities via technology. Discussion: The CoP were initiated by a group of teachers with a shared value for improving their teaching in schools and were sustained through online and offline interactions that created a safe space for sharing ideas, collaborative interactions, and learning opportunities.