Experienced teachers are, on average,more effective at raising student performance than those in their early years of teaching. This gives rise to the concern that too many teachers leave the profession after less than a full career and that too many leave troubled inner-city schools for suburban ones. Until now, the roots of these problems have not been well understood. In particular, it is not known whether teachers leave schools with high concentrations of disadvantaged and low-achieving populations for financial reasons or because of the working conditions associated with serving these students.Nor are there good estimates of what kinds of salary increases would need to be offered to slow the turnover among teachers.