Almost nobody, it seems, disputes the importance of effective teachers--including teachers themselves. However, principals actually do none of the things they do when they value something as highly as most people say they value good teachers. By looking at scores on year-end standardized tests by teachers, principals think they have a pretty good idea of who's cutting it and who isn't. This practice drives teachers wild, and for good reason. For it doesn't take into account how students performed before they entered that teacher's classroom, and it therefore penalizes those who take on students with greater challenges. 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 shows that only one in five teachers think this kind of accountability is a "poor" idea. This article discusses how to get fair and honest data about teacher effectiveness. Using value-added information rationally in the day-to-day management of public schools and classrooms will, in and of itself, have the effect of improving instruction and increasing the quality of teachers. The author presents three examples of how to use value-added information in helping teachers and managing schools. (Contains 3 notes.)