This article articulates a framework for examining the quality of formative assessment practice and provides empirical evidence in support of one of its components. Based on a study of middle school science, the study examines the accuracy of teachers' judgments of students' understanding and the relationship of such accuracy to middle school students' learning. Analyses within and between teachers show a consistent, positive relationship between teacher accuracy and student learning. Study results lend support for the power of assessment in improving student learning and also suggest some potential challenges in assuring quality formative assessment practice. (Contains 6 tables, 3 figures, and 1 footnote.)