No Child Left Behind (NCLB) legislation and public pressure to prepare students for the twenty-first century has put increasing demands on schools to improve student performance. This action research study examined the efforts of an elementary school principal and her third and fourth grade teachers to improve students' performance in math. The study employed a quasi-experimental design using mixed methods in which the researcher examined both qualitative and quantitative data and used triangulation to compare the results of findings. The research focused on how communities of practice foster change to improve rigor and engagement in math classrooms in grades three and four. The third and fourth grade teachers met in communities of practice with the goal of increasing the rigor and engagement in their math classrooms. The quantitative data consisted of results of a pre and post student survey and tallies from teacher rating charts. The qualitative data consisted of answers to the open-ended questions from the survey, teacher interviews, field notes, and data from teacher rating charts. Results indicated that the teachers and administrator perceived the intervention of the teachers meeting together to examine student work, share lessons and activities added to the engagement and rigor in the teachers' math classrooms. The use of rating charts by the teachers to examine Bloom's Taxonomy levels and types of engagement were also perceived as valuable by the participants of the study. Teacher visits to each others' classrooms was not perceived to be an effective intervention. The teachers and administrator plan to continue meeting as communities of practice using action research to select goals and interventions to improve teaching practices. [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.]