The purpose of this study was to evaluate the overall effectiveness of a three-tier model of positive behavior interventions and supports (PBIS), which was developed and tested in Head Start (HS) programs. Ten HS classrooms from five HS programs participated in the current study. Results indicated that PBIS was effective in improving classroom quality as evidenced by a statistically significant change on the classroom organization domain on the Classroom Assessment Scoring System and the overall score on the Early Childhood Environmental Rating Scale–Revised. We also found that children’s social skills on the Social Skills Rating System significantly increased from the pre- to post-assessment whereas problem behaviors on the Child Behavior Checklist decreased. The data described here are encouraging and add to the expanding database supporting the value of the three-tier model of PBIS.