The purpose of this study was to document within-year fidelity growth during installation and initial implementation of School-Wide Positive Behavioral Interventions and Supports (SWPBIS). Participants included school teams from schools throughout the United States that were in years 1 to 4 of SWPBIS implementation and routinely evaluated their implementation fidelity. The fidelity outcome was assessed with the Team Implementation Checklist (TIC) and was completed multiple times per year by SWPBIS teams. Results from multilevel fidelity growth models documented within- and between-school variability and growth predictors. Years implementing, location, school type, and enrollment size were significant predictors of beginning-year fidelity scores (intercept), and years implementing and relative socioeconomic status were significant predictors of the average rate of fidelity change per month of school (slope). These results add to the growing field of implementation science, and practice recommendations are outlined.