Formal collaborative structures for teacher teams as professional learning communities (PLC) have long been a supported practice aimed at changing instructional practice and increasing student outcomes in a cycle of continuous improvement. The purpose of this study was to evaluate the implementation of PLC teams at the middle school level in a large, urban school district and the impact of principal leadership within the implementation of the program. This program evaluation employed qualitative and quantitative approaches. Eight principals and 132 teachers representing 11 middle schools participated in an online survey about the implementation of PLC teams and their thoughts about the role of principal leadership in the program. The eight principals participated in a focus group discussion to provide additional insight from their perspective. Twelve teachers participated in a similar focus group discussion to provide their perspectives. The evaluation indicated that both principals and teachers place value in the PLC team program and are committed to improving their implementation practice. The evaluation also revealed best practices such as consistent establishment of a shared mission and values, and high levels of engagement in collaborative processes. The evaluation revealed gaps in implementation as well, especially in the areas of leadership behaviors to support PLC structures and systems around feedback and accountability for teachers. Further study should be considered including a larger evaluation that systemically looks at each individual school site and each PLC team within that school site, with the inclusion of student growth metrics. [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.]