Teachers tend to leave schools where they feel ineffective. At the same time, it's harder to be effective in schools with the lowest levels of student performance, schools that are most in need of effective teaching. There is a pressing need to improve the quality of instruction in urban schools to reduce long-standing inequities in educational performance by race and economic status. The current policy context acknowledges the importance of teaching quality for student achievement, but the most popular policy strategies for improving teaching focus on individual teachers, using incentives to attract and reward strong teachers, and developing methods to identify and remove those who are weak. Without broader work on the school as an organization, schools serving the most disadvantaged students will face high rates of teacher turnover and little chance of sustained instructional improvement. One key element in teacher retention is teachers' perceptions of their colleagues as collaborators. Teachers are more likely to stay in a school if they see themselves as part of a team that is working together toward making their school better, supported by school leadership. Teachers are also more likely to stay in schools where they feel they have influence over their work environment and they trust their principal as an instructional leader. These are the same elements of schools that are most predictive of improvements in student learning; schools that show the largest improvements in student learning over time are those where teachers work collectively on improving instruction, and where school leadership is inclusive and focused on instruction. Two further working conditions account for most of the differences in teacher mobility rates by school racial composition. One is teachers' relationships with parents. Especially in elementary schools, teachers are more likely to stay in schools where they feel that parents support their work as partners in educating students. The other, which is particularly critical in high schools, is the learning climate at the school. Teachers are more likely to stay at schools where students feel safe, and where students report that their classroom peers engage in appropriate academic behavior. (Contains 10 endnotes.)