This study examined the effects of functional assessment and positive behavior support on classroom behaviors of young children. An experimental program, entitled FACET, was implemented by school-based teams in pre-kindergarten through first-grade classrooms. Observations of child behaviors and classroom variables were compared between experimental and control classrooms. Results indicate that children in FACET classrooms made significant gains from baseline to post-intervention and exhibited a higher occurrence of positive behaviors and fewer negative behaviors compared to control children. Behavior gains were positively correlated with the level of treatment fidelity. These findings support the effectiveness of an intervention that incorporates functional assessment, collaboration, and evidence-based treatment with young children who exhibit challenging behaviors.