Teachers are the most important influence in schools on student achievement, which makes attracting and retaining excellent teachers a high priority for all school districts. But public schools in large cities like Philadelphia are especially challenged to provide every student with a highly-effective teacher. Teacher mobility--that is, transferring from one school to another or leaving the profession entirely--is disproportionately concentrated in urban school districts and has negative consequences for student performance. For these reasons, it is critical for policymakers and school leaders in Philadelphia to have a clear picture of the extent and nature of teacher mobility. This report provides evidence on teacher mobility in the School District of Philadelphia (SDP) during the 2009-10 through 2015-16 school years. Using publicly available data from the Pennsylvania Department of Education, the authors examine the extent of teacher mobility as well as the characteristics of mobile teachers and the schools that they exit and enter. For this report, teacher mobility is defined as occurring when an SDP teacher does not return to the same school in the following year. Therefore, a mobile teacher is one who moved to another SDP school, moved to a Philadelphia charter school or a Pennsylvania public school outside of Philadelphia, or left public education in Pennsylvania.