This paper assesses the determinants of teacher job change and the impact of such mobility on the distribution of teacher quality. High and low-quality teachers are more likely to leave than those in the middle of the distribution. In contrast, the relationship between teacher productivity and inter-school mobility is relatively weak. Teachers who rank above their faculty colleagues are more likely to transfer to a new school within a district and exit teaching. As the share of peer teachers with more experience, advanced degrees or professional certification increase, the likelihood of moving within district decreases. There is also evidence of assortative matching among teachers. The most effective teachers who transfer tend to go to schools whose faculties are in the top quartile of teacher quality. Teacher mobility exacerbates differences in teacher quality across schools.