Co-teaching is a service delivery model employed to establish inclusion. The lack of evidence supporting the use of co-teaching is the problem addressed. The purpose of this qualitative case study was to improve the inclusive experiences and outcomes for students by determining the role teacher self-efficacy plays in the co-teaching experiences of both general and special education teachers in a Central Illinois high school assigned to co-teach at least one class period. Vygotsky's social constructivism served as the framework because learning is social and improved when diverse learners function academically, socially, and behaviorally together; co-teaching supports this inclusion. The research questions addressed the perceptions of general and special education teachers working in a co-teaching instructional setting towards co-teaching, particularly focusing on the role and impact of self-efficacy on meeting the needs of students with disabilities. Using Likert scale questionnaires, interviews, and observations, ten co-teachers' perceptions of co-teaching and self-efficacy levels were measured and thematically analyzed using Dedoose. Observations and interviews identified a novice level of co-teaching understanding and a need for professional development in this area. This lack of understanding is a barrier to best practice implementation. Participants' perspectives directly related to observations of what was happening in classrooms and helped explain varying levels of self-efficacy. Parity was identified as important to these relationships, but little mention of the value each teacher brought to the classroom occurred. Time, supportive networks, challenging students, and previous experiences all impacted participant efficacy. Including student perception and assessment data in future research would provide improved evident to the effectiveness of co-teaching in this school. Both self and collective efficacy levels were relatively high for not having a strong understanding of co-teaching principles. These factors challenge co-taught classrooms both instructionally and mentally. [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.]