Since the release of the 1998 publication "Good Teaching Matters," the colleagues at the Education Trust have been asked to make hundreds of presentations around the country on the difference that teachers make. Virtually everywhere, the response is the same: heads nod in agreement. Almost nobody, it seems, disputes the importance of effective teachers--including teachers themselves. Perceptions that teachers are opposed to being evaluated at least in part on student achievement have stopped movement in that direction dead in its tracks. But many teachers support measures that recognize the growth students make on their watch. A large-scale teacher survey discussed in this report confirms this; it shows that only one in five teachers thinks this kind of accountability is a "poor" idea. In this issue of "Thinking K-16," the author, Kevin Carey shows how value-added data offers countless possibilities for actually getting some movement on seemingly intractable problems, for example: (1) To date, we have not figured out how to measure the effectiveness of teacher preparation programs; (2) Similarly, little is known about the effects of different models of professional development; and (3) There is the most critical agenda of all: making sure that low-income children and children of color get the teachers they need to reach state standards. This issue also contains the following articles: (1) The Real Value of Teachers (Kevin Carey); (2) Getting the Most Value out of Value-Added Data; (3) The Opportunity Gap; and (4) How States, Districts, and Schools Are using Value-Added Data. (Individual articles contain references.)