Professional development for in-service teachers is necessary to meet the changing needs of students and society. This teacher collaboration study examined the experiences of mid-career elementary teachers in Korea in their music professional development. Research questions included: (1) What were the contents of discussion? (2) What was the level of participation among elementary teachers in a collaborative group? (3) How did the focus of the group change over time? The data collection included nonparticipant observation, participants' reflective journals, researchers' logs, and artifact collection. Findings showed that participants mainly discussed teaching strategies about music classes and reflected new knowledge that were closely related to confidence in teaching. In addition, teachers in this collaborative group played different roles throughout the group meetings depending on their expertise and interests, but there was also a need for a facilitator. Finally, teachers in the group gradually identified the importance of teacher collaboration for their professional development and attempted to be peer mentors for others outside the collaborative group.