Utilizing a qualitative case-study design, this study explored how a mid-sized urban high school professional learning community was used as a reform effort to increase student achievement in mathematics on standards-based assessments. From a year-long interaction with the math professional learning community, which consisted of 3 school leaders and 6 teachers, principal leadership emerged as most significant in the professional learning community process for increasing teacher and student learning. Through detailed observations of professional learning community meetings and teachers' classrooms, this research was able to trace changes in teacher practices that resulted from the professional learning community meetings. Results indicate that school leaders may have to take responsibility for providing instructional processes and practices that are characterized by structure, pressure, and support to ensure that teacher learning and change in teacher practice that leads to improved student learning occur within professional learning communities. (Contains 2 tables and 3 footnotes.)