Professional learning strategies often rely on the belief that teaching and learning outcomes are best when teachers have a clear understanding of students’ thinking. This may seem like common sense — after all, students are active participants in the learning process, so their thinking impacts the outcome. But does research support the assumption that teachers’ knowledge of students’ thinking matters? Researchers Heather C. Hill and Mark Chin address this issue in a study recently published in the American Educational Research Journal. Hill and Chin consider what they call “incomplete evidence” in the research literature on teachers’ understanding of student thinking, including whether it can be measured reliably and how it stacks up with other areas of teacher knowledge such as subject matter content. They acknowledge