This non-experimental, descriptive study examined the effect(s), if any, that professional learning communities have had on the professional practices of secondary (grades 9-12) mathematics teachers in Boone, Clay, Putnam, and Kanawha counties in West Virginia. Also investigated were the potential differences in instructional-practice change(s) based on selected demographic variables: sex, degree level, degree level, the grade level taught, the total years of teaching experience, the total number of years in their current position, the specific math subject taught, the total number of years of PLC participation, and the composition (e.g., departmental, cross-curricular, or both) of participants' PLC. Data were collected from a 19-question researcher-adapted survey administered to 81 secondary mathematics teachers in the participating counties. Results indicated that the majority of participants had changed their instructional practice, their collaborative practice, their data study practice, and their assessment practice as a result of their participation in PLCs. [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://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]