Background: The school setting is frequently used both to educate youth about risks involved in tobacco use and to implement tobacco prevention and cessation programs. Given that school-based programs have resulted in limited success, it is necessary to identify other setting-level intervention strategies. School tobacco policies represent a type of universal intervention that might have some promise for preventing or reducing tobacco use. Methods: Hierarchical linear modeling was used to assess whether school tobacco policies were related to observations of tobacco use and current smoking among 16,561 seventh through twelfth graders attending 40 middle and high schools in Illinois. Results: Results indicated that the enforcement of school tobacco policies, but not the comprehensiveness of those policies, was associated with fewer observations of tobacco use by minors on school grounds as well as lower rates of current smoking among students. Conclusions: The school setting is a key system to impact youth tobacco use. Findings underscore the need to train school personnel to enforce school tobacco policy. (Contains 3 tables.)