The purpose of this study was to assess the perceived importance of specific contextual variables for initial implementation and sustainability of School-Wide Positive Behavior Support (SWPBS). A large, national sample of 257 school team members completed the School-Wide Universal Behavior Sustainability Index: School Teams, a research-validated measure of variables influencing sustainability of schoolwide behavior interventions. Quantitative and qualitative analyses were used to assess perceptions of the most and least important variables for initial implementation and sustainability, as well as variables perceived as more important to sustainability than initial implementation. Across quantitative and qualitative analyses, results indicated that administrator support and school team functioning were rated as the most important features for both initial implementation and sustainability, whereas barriers to SWPBS were rated as relatively less important. Staff support, integration into typical practice, and parent involvement were rated as significantly more important to sustainability than initial implementation. These results were consistent across types of raters and schools. Implications for enhancing implementation and sustainability of SWPBS are discussed.