We use detailed data from New York City to estimate how the characteristics of school principals relate to school performance, as measured by students' standardized exam scores and other outcomes. We find little evidence of any relationship between school performance and principal education and pre-principal work experience, although we do find some evidence that experience as an assistant principal at the principal's current school is associated with higher performance among inexperienced principals. However, we find a positive relationship between principal experience and school performance, particularly for math test scores and student absences. The experience profile is especially steep over the first few years of principal experience. Finally, we find mixed evidence on the relationship between formal principal training and professional development programs and school performance, with the caveat that the selection and assignment of New York City principals participating in these programs make it hard to isolate their effects. The positive returns to principal experience suggest that policies which cause principals to leave their posts early (e.g., via early retirement or a move into district administration) will be costly, and the tendency for less-advantaged schools to be run by less experienced principals could exacerbate educational inequality. Data appendix is included. (Contains 1 figure, 12 tables and 18 footnotes.)