Being an effective math educator is one part based on the quality of the tasks we give, one part how we diagnose what we see, and one part what we do with what we find. Yet with so many students and big concepts to cover, it can be hard to slow down enough to look for those moments when students' responses tell us what we need to know about next best steps. In this remarkable book, John SanGiovanni helps us value our students' misconceptions and incomplete understandings as much as their correct ones-because it's the gap in their understanding today that holds the secrets to planning tomorrow's best teaching.