Principals are widely seen as a key influence on the educational environment of schools, and nearly all principals have experience as teachers. Yet there is no evidence on whether we can predict the effectiveness of principals (as measured by their value added) based on their value added as teachers, an issue we explore using administrative data from Washington. Several descriptive features of the principal labor market stand out. First, teachers who become principals tend to have higher levels of educational attainment while teaching and are less likely to be female, but we find no significant differences in licensure test scores between those teachers who become principals and those we do not observe in the principalship. Second, principal labor markets appear to be quite localized: about 50 percent of principals previously taught in the same district in which they assumed a principalship. We find positive correlations between teacher and principal value added in reading (ELA) and similarly sized but less precise estimates in math. Teachers who become principals have slightly higher teacher value added, but the difference between the two groups is not statistically significant, suggesting that principals are not systematically selected based on their prior effectiveness when serving as a classroom teacher.