The federal mandates set forth by the Individual with Disabilities Education Improvement Act (IDEIA) and No Child Left Behind (NCLB) require an increase in inclusive education for students with special education needs in the least restrictive environment. This type of environment can be in a 'co-taught' classroom. Co-teaching is defined as an instructional delivery approach in which a regular and a special education teacher share responsibility for teaching some or all of the students assigned to a general education classroom. Given these mandates and the best way to meet students' needs, an action research study with a mixed-methods structure to collect data was used. The purpose is to investigate if professional development provided on co-teaching would help the teachers at a High School in the suburbs of Phoenix, Arizona, understand co-teaching. This was provided because co-teaching is an instructional practice used to comply with the federal mandates. Over sixteen weeks, three co-teaching teams were provided on-site professional development, resources, and support. Data were collected through the use of both quantitative (survey and checklist) and qualitative (open-ended survey questions, teacher journals, classroom observations/field notes, focus group interviews, and a member check questionnaire) measures. Quantitative analyses indicated that after trainings the participants' view on co-teaching improved in curriculum goals and modifications, interpersonal communication, and assessment. However, trainings did little to change ideas about inclusive practices. Data sources indicated participants felt inclusion caused too many students with special education needs to be in one classroom, and that teachers did not have enough training and/or say in the process. Qualitative analysis revealed the supportive co-teaching approach was used most often, but there was limited time and opportunities for collaboration, planning and decision making among the teams. My work shows that the training, resources, and support that were provided helped the teachers at this site raise their efficacy towards co-teaching. It also reveals participants' desire for additional professional development on co-teaching in order for them to continue to improve their co-teaching practice and best meet the needs of students with special education needs in the least restrictive environment. [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.]