At a time when teacher quality has emerged as a key factor in student learning, a statistical technique that determines the "value added" that teachers bring to student achievement is getting new scrutiny. Value-added measures compare students' growth in achievement to their expected growth, based on prior achievement and demographic factors over which teachers have no control. Teachers whose students exceed the expected rate of growth are considered effective, while those whose students grow in achievement at a slower rate are considered less effective. Some advocates of the approach have proposed it as a fair way of awarding teachers bonuses based on performance, and indeed some school districts, notably Houston, are using value-added methods in pay-for-performance systems. Advocates point out that, despite their limitations, value-added measures are far superior to existing measures of performance. Value-added assessment represents an improvement over existing forms of measurement, but practitioners find challenges in how it is being implemented, especially when tied to teacher incentive bonuses. (Contains 5 resources.)