Sustained implementation of effective behavior support systems and practices is critical for improving academic and behavior outcomes for students with and without disabilities. Although implementation studies have identified variables facilitating sustained implementation of Tier 1 behavior support systems in schools, little research exists examining Tier 2 and 3 behavior support systems. The purpose of this 5-year prospective study was to identify practice, school, and district variables facilitating sustained implementation of Tiers 2 and 3 behavior support systems in schools. The sample included 646 schools implementing positive behavioral interventions and supports (PBIS) across 23 U.S. states. Using structural equation modeling, we found several key variables to be predictive of sustained implementation (sustained implementation and implementation quality) of Tier 2 (Tiers 1 and 2 fidelity in Year 1 and a general Tiers 2 and 3 sustainability factor) and Tier 3 behavior support systems (Tier 1 and Tier 3 fidelity in Year 1 and a general Tiers 2 and 3 sustainability factor). Implications for how district and school leadership teams can improve sustained implementation of Tiers 2 and 3 behavior support systems are discussed.