This two-cycle Action Research study was designed to examine and improve a co-teaching program for administrators and co-teachers at the high school level in one school district in Massachusetts. Co-teaching is the intentional sharing of a classroom by two educators with the goal of providing an enriched educational experience for all learners. This study focused on ways to support the administrative team to initiate and sustain co-teaching implementation. Interviews with Cycle 1 educators and administrators revealed that much of the success of co-teaching was in administrator control. This finding informed the design of the Cycle 2 action step, which was to co-construct a co-teaching toolkit containing resources aligned with the themes from the Cycle 1 data. The toolkit and related audit tool were reviewed by three school administrative teams. Data collected from these participants through focus groups, survey, and interviews identified key findings that responded to the purpose of the research: the importance of cultivating relationships among all stakeholders, the need for administrators to build a co-teaching program for sustainability, and the necessary alignment of administrative team philosophies related to co-teaching. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]