"Responsive teaching" means something that goes beyond the images of teachers presenting information or crafting experiences, to teachers really focusing attention on student thinking. It is closely connected to assessment, not just in the sense of exams and scores but in the sense of teachers' ongoing awareness of what and how the students are doing. This chapter defines "responsive teaching," and focuses on how it serves the objective of students coming to genuine understanding of the scientific concepts. For example, students may know that air is matter but with responsive teaching, they will be able to explain why air is matter.