This descriptive phenomenology study investigated middle school mathematics teachers' formative assessment practices in the Dominican Republic. The study explored three research questions about the types of formative assessment strategies teachers implemented, how they used formative assessment data to make instructional decisions, and the challenges they faced implementing these strategies. Eighteen middle school mathematics teachers from public and private schools in three southern cities participated. The data collection consisted of semi-structured interviews, focus interviews, and reviews of mathematics lessons. The findings of this investigation indicate that teachers implemented various formative assessment strategies and took an active role in the process. Thus, teachers rarely engage students in peer assessment, self-assessment, collaborative work, and other methods that promote students' ownership of the learning process. Some teachers used formative assessment data to adjust instruction, while others provided academic intervention to students or assigned a grade. Finally, teachers experienced contextual and personal challenges implementing formative assessment, such as students' limited skills in mathematics and motivation, large class sizes, limited resources, and time to plan and implement formative assessment. Contextual factors relating to students' current performance and motivation were the most common challenges reported by teachers. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]