This study followed a causal comparative research design that utilized mixed methods. This non-experimental investigation sought to identify potential cause and effect relationships between variations in achievement data among schools. The purpose of the study was to determine if urban students' reading and math achievement increased as a result of weekly collaboration among teachers in a large, urban school district in Texas. Repeated Measures ANOVA results and pairwise comparisons from quantitative findings revealed that several statistically significant variations (p less than 0.05) existed in elementary, middle, and high school achievement and commended performance before and after Professional Learning Communities (PLCs) were established. According to the Wilks' Lambda Multivariate test, significant increases from 2006-2010 were represented in mean percent passing and commended performance for the elementary, middle, and high schools. Sixteen categories and two subcategories emerged from teachers' responses to audio-taped focus group interview questions. This qualitative data showed that teachers generally perceived that PLCs impacted their instruction and student achievement. [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.]