Professional learning communities (PLCs) are a widespread phenomenon in K-12 education, as they are perceived as a mechanism for enhancing teacher effectiveness and, therefore, student achievement. However, there is a dearth of empirical research on PLCs--particularly, the primary focus that the communities take, teachers' perceptions and attitudes toward the communities, and the relationship of the community structure and student achievement. Our research addressed this gap by surveying 145 K-12 teachers engaged in PLCs. Analysis revealed that the teachers perceived a wide range of PLC foci and were for the most part positive about their involvement in the PLCs, but we failed to find a relationship between PLCs and student achievement in state-mandated standardized teaching scores. Implications, future directions for research, and limitations are discussed. (Contains 4 tables.)