In Chapter 3, we learned that knowledge about teacher expectations and students as social beings and teaching actions like good management and time use can help principals provide teachers with useful feedback for improving teaching. Now, we discuss how knowledge of effective teaching practices and observation and feedback work jointly to improve student achievement. Consequently, this chapter addresses these questions: Does feedback matter? How and when does it matter? The questions are basic, yet the answers are essential. Recall that RttT assumes by rewarding highly effective teachers and eliminating ineffective teachers, and providing feedback on instruction, student achievement will improve. Will providing teachers with feedback about their teaching improve instruction in meaningful ways? What is known about principals’ effects on student achievement? We describe effective principal practice as measured by student achievement outcomes because one way principals achieve effects on student achievement is through instruction. Accordingly, then, we discuss what is known about how teachers modify their instruction in response to feedback. Finally, our focus narrows to the task assigned to many of our readers-observing and providing feedback to teachers. The evidence provided here is valuable but weaker than that presented in Chapter 3. One major difference is that much of the teacher effectiveness literature is based upon direct observation of teaching, whereas the literature on principals’ feedback to teachers is based on self-report or teachers’ responses to questionnaires. Research on feedback is vast, but much of it has been established in the context of teachers providing feedback to students, in experimental lab conditions utilizing college students, or in corporate settings. With that said, we make a modest attempt to summarize existing research, assessing if and how principals (and others) can use it to inform practice.