Numerous districts are implementing Professional Learning Communities (PLCs) as a part of reform efforts to improve student achievement to meet external accountability mandates. Few districts, however, have considered the essential supports and components that teachers working in PLCs require for these teams to result in instructional improvement. This study reports on the work of one small urban district that attempted to establish and support PLCs in developing essential PLC characteristics, implementing an improvement process, and establishing an instructional goal, research-based practices shown to facilitate improvement. This district also provided professional development to teachers and administrators in the implementation process. The study profiles the work of both high-functioning PLCs and struggling PLCs found at six schools in this district at the end of two years of PLC work. The work of these PLCs and the factors that supported their work are described from the teachers' viewpoint. While participation in the districtwide professional development sessions on PLCs correlates with the effectiveness of PLC work at these six schools, results of a districtwide teacher survey suggest that the role of the school leader in the process of implementing PLCs is the most critical element of a successful PLC initiative. Findings from this study affirm the research-based practices on which this district's implementation plan was based, while suggesting that schools' existing practices and professional development experiences, culture, and the readiness of school leaders be considered and analyzed prior to initiating a districtwide implementation of PLCs. The study concludes by recommending that districts consider providing differentiated supports and targeted professional development to schools during their first years of PLC work to ensure growth among all PLC teams. [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.]