Co-teaching, an instructional model serving students with special education needs in the general education classroom, has been proliferating in response to federal requirements and because it has potential in reaching all students. Implementation of co-teaching has been inconsistent and there has been little evidence co-teaching has positively impacted student achievement. Researchers have recommended providing professional development for co-teachers, but there has been scant research or agreement regarding the specific professional development necessary for co-teachers to be effective. The purpose of this phenomenological study was to explore the lived experiences of successful co-teachers and their administrators in order to gain in-depth knowledge about their experiences, toward the goal of improving co-teaching and improving student learning. In-depth, semi-structured interviews were conducted with 11 successful co-teachers, six principals, and five special education administrators, during which participants described their experiences with co-teaching. Participants were from five school districts located within three contiguous counties in New York State purposively selected for having implemented co-teaching for a minimum of five years. Transcripts were analyzed using NVivo 9 and Microsoft's Word Comment Function. The modified Van Kaam data analysis method revealed successful co-teachers and administrators had similar perceptions of successful co-teaching. They all shared the educational philosophy of inclusion and believed the co-teaching model had social and academic benefits for all students. Participants believed positive interpersonal relationships were critical to the success co-teaching and believed that relationships were built because of confidence in each other's teaching and work ethic. Co-teachers and administrators thought common planning time was critical for co-teachers but differed in their perceptions of how to utilize planning time. Administrators perceived professional development as valuable for the success of co-teaching while co-teachers minimized its importance. Administrators felt they could provide greater support for co-teaching while co-teachers described limited roles for administrators. Recommendations include researching pre- and in-service professional development, administrative support, and planning time utilization to improve co-teaching. Recommendations also include conducting research into the achievement of general and special education students who have been taught by co-teachers who embody the characteristics of successful co-teachers identified in this study. [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/en-US/products/dissertations/individuals.shtml.]