This qualitative study investigated how teacher participation in professional learning communities leads to shared expectations and understandings [among teachers] about teaching and learning within their schools, thereby establishing consistently high-quality instructional practice across classrooms. This multi-case study of four teacher teams (two middle school teams and two high school teams) from two high-performing communities examined what happens when teachers come together to collaborate. Specifically, this study sought to provide insights into why some individuals and groups are motivated to ask hard questions about problems of instruction and how particular interactions enable or obstruct changes in instructional practices. Observation, interviews, surveys, and post observation reflections were the central data collection procedures used. Findings suggest that a new perspective of leadership grounded in bureaucratic and professional strategies facilitates meaningful teacher collaboration. Findings also reveal the power of the fine-grained processes of collaboration within teacher-teams as well as the significance of goals and teacher dispositions in determining the effectiveness of teacher teams in the context of professional learning communities. [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.]