Teacher value-added measures are increasingly being analyzed by scholars and implemented by politicians. But principal value-added is at least as important, possibly more so. Much of a school’s effectiveness may be due to the principal’s choices in hiring, firing or counseling out, advising teachers on lesson plans and pedagogical techniques, encouraging cross-classroom collaboration, assigning teachers to classrooms and grades where they have the most impact, and seeking the rare evidence-based professional development course. Moreover, principals have these effects on a school-wide basis, not just within a particular classroom. On the other hand, because principals have a schoolwide effect that is mediated through the impact of teachers, and because there are other schoolwide factors in play (including selection by households into school zones), it can be extraordinarily difficult to disentangle the effects of a principal from the effects of the school and teachers. Some recent empirical work, however, has attempted to do just that with multiple layers of fixed effects. This empirical work suggests that there is wide variation in principal quality, that principal quality may be positively related to experience, and that there may be inequality in the distribution of good principals.