In this study, I explored professional learning community sustainability at the school, district, and state levels. Researchers have found that professional learning communities are a means of effective professional learning for teachers and a vital factor in increased student achievement. However, there is very little multi-case research involving the sustainability of professional learning communities. This multi-site case study extends the research on this topic. I used qualitative methods to clarify my understanding of the phenomenon of sustainability of PLC communities. I investigated how the PLC members' stances and actions, perceptions of school culture, and commitment to lifelong learning impacted a PLC's effectiveness and sustainability as did leadership style. I used a snowball data collection technique to collect data from 35 participants at the school-level, district-level, or state-level PLC. I interviewed each of the participants using a 14-question questionnaire, reviewed document artifacts, and analyzed the collected data. I found that professional learning communities can be sustained when a PLC's structure is collaborative and focused on learning, leadership is shared and distributed throughout the school, and supportive conditions are in place for members to engage in lifelong professional learning. The school-level PLC was sustained because of these factors. The state-level PLC exhibited more sustainable characteristics than the district-level PLC. [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.]