The purpose of this study was to examine interactions among in-service music teachers in a graduate music teacher education program and their influence on professional growth. To collect data for this research, five different collection methods were used, including participant observation, surveying, artifact collection, nonparticipant observation, and individual interviews. Results of this study showed that graduate classes in Intensive Summer Teacher Education Program (INSTEP) evolved into communities of practice (Wenger, 1998). "The domain" indicated that the graduate students wanted to become better music teachers, so they were open to new ways of thinking and ideas regarding music, teaching, education, and music education. Based on this common goal, the graduate students understood that these class materials would be helpful for their teaching and students, so they actively participated in interactions and collaborations among classmates and shared their own knowledge and teaching stories ("the practice"). Several factors such as the commonality of working as in-service music teachers, similar teaching areas, and closeness in age helped form closer relationships from the beginning of the program. Also, through the cohort structure, intensive classes, and group projects, INSTEP students continuously interacted with other colleagues. This regular interaction in summer INSTEP classes ("the community") enabled a deeper level of community, referred to as communities of practice. Interaction and collaboration outside the classroom during and between summers or after completion of the program, however, only evolved into a community of interest (Henry & Puldelko, 2003) due to the weak social bond and low gathering's intentionality. INSTEP classes as communities of practice greatly affected students' professional development while communities of interest outside the classroom had less impact on students' learning and quality of teaching. While interacting and collaborating with classmates, exchanging personal teaching stories, and receiving support and suggestion from others, INSTEP students strengthened their understanding about class materials, confirming that interaction and collaboration among INSTEP students in INSTEP communities of practice positively and greatly affected their professional development. Recommendations for music teacher educators and future research are discussed. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]