This article describes qualitative and quantitative assessment based on responses of 221 teachers from nine elementary schools in two districts (urban and suburban) to inform plans for reducing and preventing student aggression. Teachers' perceptions of students' aggressive behavior and beliefs were validated against students' self-reports and archival disciplinary data. Using a brief survey, we found district- and grade-level differences in teachers' perceptions of students' aggressive behavior and aggression supporting cognitions. Teacher reports on these two constructs each uniquely predicted teacher perceptions of the degree to which student aggression interferes with their jobs. Focus-group interviews with teachers were used to elaborate on individual-cognitive and ecological school factors related to student aggression, including procedures for handling aggression. The importance of teacher reports is highlighted, and implications for school program development are considered. (Contains 1 table and 1 footnote.)