This article discusses the evidence for intervention validity of Functional Behavior Assessment (FBA) in designing support for students with intensive behavioral needs. Since its inclusion into the Individuals With Disabilities Education Act nearly a decade ago, FBA has been the subject of significant research investigating its use and applicability to school-based behavior support. This research, specifically designed to address the question of intervention validity, provides solid evidence of the treatment utility of FBA for behavior support, demonstrating the critical need to use a function-based approach for improved behavior outcomes. The authors present recommendations for research-validated practice in school settings and identify a research agenda to move the field forward in light of the current focus on response to intervention. (Contains 1 figure.)