Teacher attrition has attracted considerable attention as federal, state and local policies- intended to improve student outcomes, increasingly focus on recruiting and retaining more qualified and effective teachers. But policy makers are often frustrated by the seemingly high rates of attrition among teachers earlier on in their careers. This paper analyzes attrition patterns among teachers in New York City elementary and middle schools and explores whether teachers who transfer among schools, or leave teaching entirely, are more or less effective than those who remain. Findings show first-year teachers who are less effective in improving student math scores have higher attrition rates than do more effective teachers. This raises important questions about current retention and transfer policies.