Public school districts in large cities like Philadelphia are especially challenged to provide every school with an effective school leader. Principal mobility--that is, transferring from one school to another or leaving the principalship entirely--is disproportionately concentrated in school districts serving more low-income students 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 principal mobility. This report provides evidence on principal mobility among traditional public schools (TPS) in the School District of Philadelphia (SDP) and charter public schools located in Philadelphia during the 2007-08 through 2015-16 school years. The study uses individual-level data on all principals in traditional and charter schools provided by the Pennsylvania Department of Education. The authors describe the characteristics of principals in Philadelphia, the extent of principal mobility, and the characteristics of mobile principals and the schools that they exit. For this report, principal mobility is defined as occurring when a traditional or charter school principal does not return to the same school the following year. By this definition, a mobile principal is one who moved to another school in Philadelphia (either a traditional or charter school), moved to a Pennsylvania public school outside of Philadelphia, exited the principalship (i.e., was employed in another role in education), or left public education in Pennsylvania.