Functional behavior assessment (FBA) is an integral component of a positive behavior support approach to preventing problem behavior across all students in the school. As primary prevention, FBA is a collaborative school-wide practice to predict common problems and to develop school-wide interventions. As secondary prevention, FBA involves simple and realistic team-driven assessment and intervention strategies aimed at students with mildly challenging behaviors. As tertiary prevention, FBA is complex, time-consuming, and rigorous--aimed at students for whom all previous intervention attempts have been unsuccessful. Whereas the concepts of prediction, function, and prevention remain constant at all levels of positive behavior support, the considerations for and form of FBA may vary greatly. The authors present the application of FBA practices at each of the three levels of a system of positive behavior support.