The problem in this study was students failing to meet state-mandated mathematics proficiency. The purpose of this basic qualitative study was to examine teachers' experiences using formative assessment practices (FAP) in teaching mathematics in Grades 6 through 8. A gap in the literature related to teachers' use of FAP to increase mathematics proficiency. The rationale for this study was to understand teachers' use of FAP in teaching mathematics. A basic qualitative research methodology was used to conduct the study. The theoretical framework included the theory of formative assessment, learning progression theory, and learner equity theory (or right of the learner theory). The overall research question examined how middle school teachers designed and implemented FAP in teaching mathematics. Two sub-research questions investigated how teachers clarified students' mathematical misconceptions using FAP; and how teachers responded to students' ideas, providing feedback. The population and sample for this study included all middle school mathematics teachers in the state, with a sample of 12 certified mathematics teachers, with three or more years of experience and professional development training in FAP. Data collection included open-ended interviews and document reviews. Data analysis followed a thematic analysis process. Findings consistent with the literature included themes such as designing FAP to (a) improve students' engagement, (b) measure mastery of skills, (c) uncover misconceptions by diverging tasks, (d) provide feedback, and (e) encourage engagement in deep thinking at the beginning, middle, and end of lessons. Recommendations for comprehensive school district implementation and future studies with a larger population were made. [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.]