This article reports on the results of a mixed method case study investigating the relationship between professional learning community (PLC) activities and teacher improvement in a first-year middle school. Data were collected from core academic sixth, seventh, and eighth grade teachers using a professional development survey, teacher interviews, and a review of school documents. Results demonstrated that professional learning community activities--that comprised same-subject, same-grade teacher teams--had the potential to achieve significant improvements in teaching effectiveness, but this effectiveness depended on a number of factors. Contributing factors included leadership and organizational practices, the substantive details of PLC activity meetings, the nature of conversations in PLC activities, and the development of community among PLC teams. (Contains 3 tables and 1 figure.)