Most studies that have fueled alarm over the attrition and mobility rates of teachers have relied on proxy indicators of teacher quality, even though these proxies correlate only weakly with student performance. This paper examines the attrition and mobility of early-career teachers of varying quality using value-added measures of teacher performance. Unlike previous studies, this paper focuses on the variation in these effects across the effectiveness distribution. On average, more effective teachers tend to stay in their initial schools and in teaching. But the lowest performing teachers, who are generally the most likely to transfer between schools, appear to "churn" within the system, and teacher mobility appears significantly affected by student demographics and achievement levels. (Contains 5 tables and 41 footnotes.)