Research on teacher professional learning has shown that formative assessment can improve student learning more than most instructional practices. Empirical evidence indicates that thoughtfully implemented formative assessment practices improve students' learning, increase students' scores, and narrow achievement gaps between low-achieving students and others. Practiced well, formative assessment holds promise for fostering equity. Given the role that students' achievement in middle school mathematics classes can play in college-going trajectories, articulating how formative assessment can support equity in mathematics classrooms is critically important. But what is formative assessment, or FA? Does "doing FA" mean giving more quizzes, managing interim test data, or processing exit slips? Doing formative assessment in mathematics classes depends greatly on teachers' and students' use of language: producing language, taking language in, and sharpening language skills. Some conceptualizations of formative assessment are more explicit than others in their focus on language use. In this article, the authors present their conceptualization of formative assessment; introduce FA moves of priming, posing, pausing, probing, bouncing, tagging, and binning; and illustrate the moves at play in one middle school mathematics class. Each of these moves lends itself to sustaining a focus on the development of academic language for all students, which is critical to fostering equity in mathematics learning and teaching.