Co-teaching occurs when a special educator and a general education teacher teach together in an inclusive classroom. Co-teaching has been adopted across the United States as the solution to meet the legal mandates of No Child Left Behind and the Individuals with Disabilities Improvement Act. These pieces of legislation require schools to have highly qualified teachers, be accountable for student outcomes, provide access to the general education curriculum to all students, and include students with disabilities in the least restrictive setting. Despite frequent use of the co-teaching service model, most research has focused on the various models, necessary inputs for implementation, and outcomes of co-teaching, but not how co-teachers use assessment information in their teaching practices. The purpose of this qualitative multiple case study was to explore how assessment data was being used in co-teaching. The study focused on two main objectives: (a) to examine what data was collected by co-teachers and how that assessment data was used to contribute to the classroom outcomes for students with special needs; and (b) to examine if and how assessments were used to improve the practice of co-teaching. Six general education-special education co-teacher pairs and their building administrators in five Iowa schools were interviewed and additional data gathered through a focus group of co-teacher trainers, inspection of documents, and classroom observations. All the teachers had taken co-teaching training, so they had the skills required for proper implementation of co-teaching. Findings from the research study showed that assessment data was collected in all classrooms and used by co-teachers to evaluate student progress and inform instruction. Co-teachers used data to evaluate and improve their own instructional skills, as well as to inform the school system about student achievement and trends. Participants did not feel assessment data was used to impact co-teaching practice. Systems were often available to evaluate co-teaching data, but more time and information were needed supports. Future research should focus on using data system capabilities to investigate co-teaching effectiveness, the impact data teams' findings have on co-teaching, and the administrative supports provided to co-teachers. [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.]