U. S. educational policies focused on the use of high-stakes standardized achievement tests and top-down accountability to improve student achievement have had an insignificant impact. Furthermore, the focus on once-a-year standardized tests to measure student success negatively impacts daily classroom instruction and assessment practices. However, when classroom teachers consistently and effectively implement the formative assessment process in mathematics instruction, it has a positive impact on student learning. The formative assessment process is a complex pedagogical practice, and traditional professional development models do not support classroom implementation in a way that impacts student achievement. Therefore, there is a need to develop effective and sustainable training models that support teachers in implementing formative assessment processes into classroom practice. The purpose of this qualitative embedded single-case study was to explore how participants describe changes in formative assessment knowledge and classroom practices after participating in a learner-centered professional development. The formative assessment process is difficult for teachers to implement, and using a qualitative embedded single-case study design allowed me to investigate a complex phenomenon in a real-world context. Four elementary school teachers from different grade levels engaged in a six-week professional development program on the formative assessment process in mathematics instruction. The theoretical framework based on the constructivist theory of learning (Dewey, 1938) guided the research questions, as well as the data collection and analysis. I collected and analyzed qualitative data using observations, interviews, and artifacts. The findings revealed that participants perceived certain components of the professional development program supported their application of formative assessment knowledge into classroom practice. As a result of participating in the professional development program, participants developed a new understanding of the purpose and importance of the formative assessment process within mathematics instruction. Furthermore, teachers felt more confident and successful with their mathematics instruction when they planned and focused on the formative assessment process. The findings provide evidence for districts and instructional leaders about the design of effective professional development models that support teachers in their application of formative assessment knowledge into classroom practice. [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/en-US/products/dissertations/individuals.shtml.]