This straightforward statement by John Hattie and Helen Timperley is hugely important, and sets the agenda for this chapter. Feedback is part of everyday life and takes many forms, for example a comment from a family member, friend or colleague, the reading on the bathroom scales or the response to a job application. A few moments’ reflection will bring to mind the variety of feelings generated by receiving feedback, and its effects and consequences. In education feedback is generally considered ‘a good thing’, although this is not necessarily the case. Feedback is powerful, but since its effects can be for good or ill we need to consider it carefully, strive to understand its complexities and nuances, and seek to ensure that feedback supports rather than frustrates our educational aims.