Several years ago, I was working with a group of high school math teachers. Their assistant principal was impressed with their practice of sharing data from common assessments, assuming that they used these data to drive instruction. However, when I asked the teachers which data they used when teaching, they said that student work and questions during class were much more valuable. Apparently, people may interpret “data-driven instruction” differently. As a mathematics teacher, what data can you collect, and how can you use those data to improve instruction?