In this dissertation, I aimed to improve elementary teachers' mathematical instruction by enhancing their content knowledge and skills through the implementation of professional learning communities (PLCs) and professional development. The research design consisted of multiple evaluation steps conducted over time with a collaborative action plan the leadership team, mathematics specialist, and teachers developed. Quantitative and qualitative data were collected from participating classrooms and were analyzed at different phases of the program evaluation. The scope of the study addressed the lack of content knowledge among elementary teachers, which led to gaps in instruction and difficulties in explaining to students advanced mathematical concepts. The research justified a mixed methods approach to capture accurately the effectiveness of the interventions and the use of surveys, observations, anecdotal notes, and various quantitative data sources (e.g., common assessments, state test data, and benchmark data). A control group was used to measure the impact of the applied research action plan. The results indicated positive changes in teachers' perceptions of the effectiveness of PLCs in addressing instructional challenges. Improvements were observed in collegial support, belief in every student's ability to learn, ease of communication within the PLC, and the impact of PLC time on daily instruction. Observations and learning walks supported the conclusion of the mathematics PLCs and professional development initiatives to address effectively instructional challenges in classrooms. The teachers who participated in mathematics professional development demonstrated increased frequency of high-quality instructional practice (HQIP). They exhibited enhanced self-efficacy, used manipulatives, engaged in regular practice, facilitated understanding of word problems, provided small group instruction, used data to drive instruction, and used resources effectively. Overall, participating in mathematics PLCs and professional development appears to affect positively instructional practices and student achievement in elementary mathematics. [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.]