This study investigated the relationships between teacher collaboration practices known as working as a professional learning community (PLC) and student performance. Through a review of the current literature, an operational framework of PLCs was developed that distinguished results-oriented from inquiry-oriented PLCs. The study considered the relationships between the tasks associated with these PLCs and student performance as measured by the achievement and growth scores of seventh grade English language arts students on Massachusetts's spring 2012 state mandated standardized assessment. A correlational analysis was conducted to examine the relationship between PLC practices and student performance and a multiple regression analysis was conducted to examine the relationships between the explanatory variables of PLC practices and the response variables of student performance. To obtain data about the tasks taking place in PLCs, all Massachusetts middle schools possessing a seventh grade English language arts (ELA) program with 220 or more students were surveyed. When combined with other independent variables in multiple regressions, the following statistically significant and predictive relationships were found: (1) the frequency of school-level English department common assessments was positively predictive of state assessment student growth scores; (2) the amount of collaboration time teachers spent analyzing common assessment results and discussing instructional practice informed by these results was negatively predictive of state assessment student growth scores; (3) increases in teacher autonomy over how collaboration time was used was positively predictive of state assessment achievement scores; and (4) the amount of collaboration time teachers spent working on tasks other than highly structured results-oriented ones was negatively predictive of state assessment achievement scores. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]