What are the characteristics of formal formative assessments that are both effective in improving student learning and an efficient use of a teacher's time and efforts? That's the question that the authors explore in this article drawing on a five-year research study. First, formal formative assessment is defined as being planned in advance, designed with the purpose of gathering information from all students in the class at the same time, and intended to move students forward through feedback and/or instructional adjustments. These activities should be windows into students' thinking--revealing not just answers to questions, but also how students arrived at those answers. The writers argue that what teachers do with formal formative assessments is especially important. Instead of leaving low-level comments on work, such as "good" and "not quite," teachers should provide meaningful descriptive and prescriptive comments that help students understand their strengths or mistakes and how they can improve. Finally, teachers should use the results of these assessments to inform their next instructional steps, whether that is deciding to reteach a concept to a small group, meeting individually with one student, or introducing the whole class to a new concept entirely.