Teaching matters. Study after study confirms that students who have high quality teachers make significant and lasting achievement gains. Those with less effective teachers play a constant game of academic catch-up. Having high quality teachers consistently tops parents' list of critical elements of an effective school. As the National Commission on Teaching and America's Future stated, "What teachers know and can do makes the crucial difference in what students learn." But what is a "good" teacher? And how is effective teaching measured? The real question is, "What do teachers contribute to student learning and how do we know?" In other words, is there a way to use student test scores to determine how much of student learning can be attributed to a particular teacher? In fact, there is a method that has been proposed. Pioneered in Tennessee, it is called "value-added." It is called this because it is meant to be a measure of the value teachers add to students' learning. Using this method requires some complicated analysis of statistics, but the end result is considered to be useful information--based on test scores--about teachers' contribution to student learning. Also in this system, each student is compared to his or her own performance the previous year, so other issues (such as family income) are not involved in the analysis. Tennessee's results from this system are noteworthy. (Contains a list of 7 resources.) [This article was produced by the Center for Assessment and Evaluation of Student Learning (CAESL) at WestEd.]