School choice policies are often based on the idea that competition will generate better outcomes for all students. Yet there is limited empirical research about how school leaders actually perceive competition and whom they view as rivals. Drawing on concepts from economic sociology, I study principals' competitive networks and the sets of schools they view as rivals, and I use network and statistical analysis to explore factors that explain the existence of a competitive tie between two schools. Most school leaders perceived some competition, but the extent to which they competed with other schools varied significantly. Factors that predicted a competitive relationship between two schools included geography, student transfers, school performance, principal characteristics, and charter network.