Using a theoretical model developed from recent research on organizational behavior and professional development, the purpose of this concurrent triangulation, mixed method case study was to describe in detail the relationship between professional learning community activities and teacher improvement for core middle school teachers in a first year school adopting DuFour's (2004b) professional learning community principles. Specifically, the study focused on three areas: the features of professional learning community activities that exhibited a relationship to changes in teachers' content and pedagogical knowledge and skills, along with changes in teachers' instructional practices; the efficacy of professional learning community activities in relation to teacher grade level, subject area, and years of teaching experience; and organizational and leadership factors that influenced the efficacy of professional learning community activities.

The study used a case study format and focused on a first-year middle school that had incorporated DuFour's (2004b) professional learning community principles. After selecting an appropriate test site using a set protocol, the study relied on three types of data. Quantitative data focusing on the nature of professional learning community activities were collected from core academic 6th, 7th, and 8th grade teachers using Garet et al.'s (1999) Teacher Activity Survey. Qualitative data were collected from interviews with a purposefully selected group of ten teachers and from a review of school documents.

A comparative analysis of quantitative and qualitative data indicated that significant differences existed between grade levels in terms of the impact of professional learning community activities on teacher improvement, and that 6^th and 7^th grade teachers exhibited high degrees of professional improvement as a result of participation in PLC activities. The efficacy of professional learning community activities depended on a number of factors, including 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.