This study evaluated the impact of the implementation of a professional learning community known as Critical Friends Groups (CFGs) on teachers' perceptions of their school as a professional learning community, on professional growth and development, instructional practices, and student achievement. This research utilized a mixed-methodology design which included both quantitative and qualitative data. Using pre- and posttest surveys, teacher interviews, and student achievement data, the researcher was able to triangulate the data to obtain a holistic understanding of the effects of CFGs. The data revealed CFGs had a significant impact on teachers' perceptions of their school as a professional learning community, on professional growth and development, and instructional practices. The data were inconclusive as to the impact of CFGs on student achievement. [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.]