Purpose: The purpose of this qualitative case study was to determine the impact co-teaching professional development has had on teacher attitude and classroom practices in a secondary, middle school setting. Methodology: This qualitative case study design included the use of best practice checklists and personal interviews. Educators from a middle school in the California Bay Area were the target population for the case study. Educators were asked to rate their current co-teaching practice and respond to a series of interview questions. Limitations of the study included sample size and population. Other general limitations included respondents' answering the checklists and interview questions with fidelity and the extent to which collected research findings can be generalized to other districts and schools. Findings: Findings of this study reinforce previous research on co-teaching, including the components for effective co-teaching relationships, common planning time, administrative support, and similar teacher pedagogy. Findings concluded that the professional development had a positive impact on educator attitude and co-teaching practices. The professional development served to remove common barriers identified by research and the participants of this study. The analysis of best practices checklists showed general education teachers rated themselves lower in the participation of co-teaching practices than both special education teachers and administrators. Conclusions: This study contributed to the research by identifying professional development that had an impact on educator attitude and classroom practice. Educational leaders can use this model to build new or improve existing co-teaching professional development opportunities through consideration of the components and strategies presented in this study. Recommendations: Recommendations for future research include the duplication of the research design in order to explore the impact of professional development on any number of district or school initiatives. Future inquiries may explore identifying the key personality types most prevalent among successful co-teachers. Teacher preparation programs could be evaluated to determine if teachers are adequately prepared for engaging in co-teaching partnerships. [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.]