The purpose of this action research study was to develop and implement common formative assessments for mathematics teachers to inform instructional practices. To address the purpose of this action research study, the following research questions guided this study: 1. How can the district instructional leadership team support professional learning for math teachers to develop and implement common formative assessments to inform instructional practices?; 2. How do mathematics teachers describe the impact of professional learning on their capacity to utilize common formative assessments as a tool to inform instructional practices?; and 3. What do Action Research Design Team (ARDT) members learn in the process of leading the design and implementation of common formative assessments in mathematics? To address the research questions, the instructional leadership team designed and facilitated professional learning sessions on developing and implementing common formative assessments to inform the instructional practices of mathematics teachers. The following themes emerged from the study: 1) Cyclical and relevant professional learning models desired instructional practice; 2) Time, relevance, and reflection are key; and 3) Clarity in the framework, processes, and expectations positively influence implementation. The findings illustrated that instructional leaders have the ability to impact collaborative planning in the development and implementation of common formative assessments, teacher efficacy in the implementation of the assessment, and reflective discussion in the analysis of the assessment results. [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.]